# Tips for golfing in x86/x64 machine code

I noticed that there's no such question, so here it is:

Do you have general tips for golfing in x86/x64 machine code? If the tip only applies to a certain environment or calling convention, please specify that in your answer.

# Choose your calling convention to put args where you want them.

The language of your answer is asm (actually machine code), so treat it as part of a program written in asm, not C-compiled-for-x86. Your function doesn't have to be easily callable from C with any standard calling convention. That's a nice bonus if it doesn't cost you any extra bytes, though.

In a pure asm program, it's normal for some helper functions to use a calling convention that's convenient for them and for their caller. Such functions document their calling convention (inputs/outputs/clobbers) with comments.

In real life, even asm programs do (I think) tend to use consistent calling conventions for most functions (especially across different source files), but any given important function could do something special. In code-golf, you're optimizing the crap out of one single function, so obviously it's important/special.

To test your function from a C program, you can write a wrapper that puts args in the right places, saves/restores any extra registers you clobber, and puts the return value into e/rax if it wasn't there already.

### The limits of what's reasonable: anything that doesn't impose an unreasonable burden on the caller:

• esp/rsp must be call-preserved1; other integer regs are fair game for being call-clobbered. (rbp and rbx are usually call-preserved in normal conventions, but you could clobber both.)

• Any arg in any register (except rsp) is reasonable, but asking the caller to copy the same arg to multiple registers is not.

• Requiring DF (string direction flag for lods/stos/etc.) to be clear (upward) on call/ret is normal. Letting it be undefined on call/ret would be ok. Requiring it to be cleared or set on entry but then leaving it modified when you return would be weird.

• Returning FP values in x87 st0 is reasonable, but returning in st3 with garbage in other x87 registers isn't. The caller would have to clean up the x87 stack. Even returning in st0 with non-empty higher stack registers would also be questionable (unless you're returning multiple values).

• Your function will be called with call, so [rsp] is your return address. You can avoid call/ret on x86 using a link register like lea rbx, [ret_addr]/jmp function and return with jmp rbx, but that's not "reasonable". That's not as efficient as call/ret, so it's not something you'd plausibly find in real code.

• Clobbering unlimited memory above rsp is not reasonable, but clobbering your function args on the stack is allowed in normal calling conventions. x64 Windows requires 32 bytes of shadow space above the return address, while x86-64 System V gives you a 128 byte red-zone below rsp, so either of those are reasonable. (Or even a much larger red-zone, especially in a stand-alone program rather than function.)

Note 1: or have some well-defined sensible rule for how RSP is modified: e.g. callee-pops stack args like with ret 8. (Although stack args and a larger ret imm16 encoding are usually not what you want for code-golf). Or even returning an array by value on the stack is an unconventional but usable calling-convention. e.g. pop the return address into a register, then push in a loop, then jmp reg to return. Probably only justifiable with a size in a register, else the caller would have to save the original RSP somewhere. RBP or some other reg would have to be call-preserved so a caller could use it as a frame pointer to easily clean up the result. Also probably not smaller than using stos to write an output pointer passed in RDI.

Borderline cases: write a function that produces a sequence in an array, given the first 2 elements as function args. I chose to have the caller store the start of the sequence into the array and just pass a pointer to the array. This is definitely bending the question's requirements. I considered taking the args packed into xmm0 for movlps [rdi], xmm0, which would also be a weird calling convention and ever harder to justify / more of a stretch.

### Return a boolean in FLAGS (condition codes)

OS X system calls do this (CF=0 means no error): Is it considered bad practice to use the flags register as a boolean return value?.

Any condition that can be checked with one jcc is perfectly reasonable, especially if you can pick one that has any semantic relevance to the problem. (e.g. a compare function might set flags so jne will be taken if they weren't equal).

### Require narrow args (like a char) to be sign or zero extended to 32 or 64 bits.

This is not unreasonable; using movzx or movsx to avoid partial-register slowdowns is normal in modern x86 asm. In fact clang/LLVM already makes code that depends on an undocumented extension to the x86-64 System V calling convention: args narrower than 32 bits are sign or zero extended to 32 bits by the caller.

You can document/describe extension to 64 bits by writing uint64_t or int64_t in your prototype if you want, e.g. so you can use a loop instruction, which uses the whole 64 bits of rcx unless you use an address-size prefix to override the size down to 32 bit ecx (yes really, address-size not operand-size).

Note that long is only a 32-bit type in the Windows 64-bit ABI, and the Linux x32 ABI; uint64_t is unambiguous and shorter to type than unsigned long long.

### Existing calling conventions:

• Windows 32-bit __fastcall, already suggested by another answer: integer args in ecx and edx.

• x86-64 System V: passes lots of args in registers, and has lots of call-clobbered registers you can use without REX prefixes. More importantly, it was actually chosen to allow compilers to inline (or implement in libc) memcpy or memset as rep movsb easily: the first 6 integer/pointer args are passed in rdi, rsi, rdx, rcx, r8, and r9.

If your function uses lodsd/stosd inside a loop that runs rcx times (with the loop instruction), you can say "callable from C as int foo(int *rdi, const int *rsi, int dummy, uint64_t len) with the x86-64 System V calling convention". example: chromakey.

• 32-bit GCC regparm: Integer args in eax, ecx, and edx, return in EAX (or EDX:EAX). Having the first arg in the same register as the return value allows some optimizations, like this case with an example caller and a prototype with a function attribute. And of course AL/EAX is special for some instructions.

• The Linux x32 ABI uses 32-bit pointers in long mode, so you can save a REX prefix when modifying a pointer (example use-case). You can still use 64-bit address-size, unless you have a 32-bit negative integer zero-extended in a register (so it would be a large unsigned value if you did [rdi + rdx], going outside the low 32 bits of address space.).

Note that push rsp/pop rax is 2 bytes, and equivalent to mov rax, rsp, so you can still copy full 64-bit registers in 2 bytes.

• When challenges ask to return an array, do you think returning on the stack is reasonable? I think that's what compilers will do when returning a struct by value.
– qwr
May 18, 2018 at 18:52
• @qwr: no, the mainstream calling conventions pass a hidden pointer to the return value. (Some conventions pass/return small structs in registers). C/C++ returning struct by value under the hood, and see the end of How do objects work in x86 at the assembly level?. Note that passing arrays (inside structs) does copy them onto the stack for x86-64 SysV: What kind of C11 data type is an array according to the AMD64 ABI, but Windows x64 passes a non-const pointer. May 18, 2018 at 20:14
• so what do you think about reasonable or not? Do you count x86 under this rule codegolf.meta.stackexchange.com/a/8507/17360
– qwr
May 18, 2018 at 23:37
• @qwr: x86 isn't a "stack based language". x86 is a register machine with RAM, not a stack machine. A stack machine is like reverse-polish notation, like x87 registers. fld / fld / faddp. x86's call-stack doesn't fit that model: all normal calling conventions leave RSP unmodified, or pop the args with ret 16; they don't pop the return address, push an array, then push rcx / ret. The caller would have to know the array size or have saved RSP somewhere outside the stack to find itself. May 18, 2018 at 23:56
• Call push the address of instruction after the call in the stack jmp to function called; ret pop the address from the stack and jmp to that address
– user58988
Mar 12, 2019 at 18:56

## mov-immediate is expensive for constants

This might be obvious, but I'll still put it here. In general it pays off to think about the bit-level representation of a number when you need to initialize a value.

b8 00 00 00 00          mov    $0x0,%eax  should be shortened (for performance as well as code-size) to 31 c0 xor %eax,%eax  ### Initializing eax with -1: b8 ff ff ff ff mov$-1,%eax


can be shortened to

31 c0                   xor    %eax,%eax
48                      dec    %eax


or

83 c8 ff                or     $-1,%eax  Or more generally, any 8-bit sign-extended value can be created in 3 bytes with push -12 (2 bytes) / pop %eax (1 byte). This even works for 64-bit registers with no extra REX prefix; push/pop default operand-size = 64. 6a f3 pushq$0xfffffffffffffff3
5d                      pop    %rbp


Or given a known constant in a register, you can create another nearby constant using lea 123(%eax), %ecx (3 bytes). This is handy if you need a zeroed register and a constant; xor-zero (2 bytes) + lea-disp8 (3 bytes).

31 c0                   xor    %eax,%eax
8d 48 0c                lea    0xc(%eax),%ecx

• BTW to initialize a register to -1, use dec, e.g. xor eax, eax; dec eax Jul 18, 2017 at 10:28
• @anatolyg: 200 is a poor example, it doesn't fit in a sign-extended-imm8. But yes, push imm8 / pop reg is 3 bytes, and is fantastic for 64-bit constants on x86-64, where dec / inc is 2 bytes. And push r64 / pop 64 (2 bytes) can even replace a 3 byte mov r64, r64 (3 bytes with REX). See also Set all bits in CPU register to 1 efficiently for stuff like lea eax, [rcx-1] given a known value in eax (e.g. if need a zeroed register and another constant, just use LEA instead of push/pop Mar 29, 2018 at 13:35

In a lot of cases, accumulator-based instructions (i.e. those that take (R|E)AX as the destination operand) are 1 byte shorter than general-case instructions; see this question on StackOverflow.

• Normally the most useful ones are the al, imm8 special cases, like or al, 0x20 / sub al, 'a' / cmp al, 'z'-'a' / ja .non_alphabetic being 2 bytes each, instead of 3. Using al for character data also allows lodsb and/or stosb. Or use al to test something about the low byte of EAX, like lodsd / test al, 1 / setnz cl makes cl=1 or 0 for odd/even. But in the rare case where you need a 32-bit immediate, then sure op eax, imm32, like in my chroma-key answer Mar 29, 2018 at 13:31

Use special-case short-form encodings for AL/AX/EAX, and other short forms and single-byte instructions

Examples assume 32 / 64-bit mode, where the default operand size is 32 bits. An operand-size prefix changes the instruction to AX instead of EAX (or the reverse in 16-bit mode).

• inc/dec a register (other than 8-bit): inc eax / dec ebp. (Not x86-64: the 0x4x opcode bytes were repurposed as REX prefixes, so inc r/m32 is the only encoding.)

8-bit inc bl is 2 bytes, using the inc r/m8 opcode + ModR/M operand encoding. So use inc ebx to increment bl, if it's safe. (e.g. if you don't need the ZF result in cases where the upper bytes might be non-zero).

• scasd: e/rdi+=4, requires that the register points to readable memory. Sometimes useful even if you don't care about the FLAGS result (like cmp eax,[rdi] / rdi+=4). And in 64-bit mode, scasb can work as a 1-byte inc rdi, if lodsb or stosb aren't useful.

• xchg eax, r32: this is where 0x90 NOP came from: xchg eax,eax. Example: re-arrange 3 registers with two xchg instructions in a cdq / idiv loop for GCD in 8 bytes where most of the instructions are single-byte, including an abuse of inc ecx/loop instead of test ecx,ecx/jnz

• cdq: sign-extend EAX into EDX:EAX, i.e. copying the high bit of EAX to all bits of EDX. To create a zero with known non-negative, or to get a 0/-1 to add/sub or mask with. x86 history lesson: cltq vs. movslq, and also AT&T vs. Intel mnemonics for this and the related cdqe.

• lodsb/d: like mov eax, [rsi] / rsi += 4 without clobbering flags. (Assuming DF is clear, which standard calling conventions require on function entry.) Also stosb/d, sometimes scas, and more rarely movs / cmps.

• push/pop reg. e.g. in 64-bit mode, push rsp / pop rdi is 2 bytes, but mov rdi, rsp needs a REX prefix and is 3 bytes.

xlatb exists, but is rarely useful. A large lookup table is something to avoid. I've also never found a use for AAA / DAA or other packed-BCD or 2-ASCII-digit instructions, except for a hacky use of DAS as part of converting a 4-bit integer to an ASCII hex digit, thanks to Peter Ferrie.

1-byte lahf / sahf are rarely useful. You could lahf / and ah, 1 as an alternative to setc ah, but it's typically not useful.

And for CF specifically, there's sbb eax,eax to get a 0/-1, or even un-documented but universally supported 1-byte salc (set AL from Carry) which effectively does sbb al,al without affecting flags. (Removed in x86-64). I used SALC in User Appreciation Challenge #1: Dennis ♦.

1-byte cmc / clc / stc (flip ("complement"), clear, or set CF) are rarely useful, although I did find a use for cmc in extended-precision addition with base 10^9 chunks. To unconditionally set/clear CF, usually arrange for that to happen as part of another instruction, e.g. xor eax,eax clears CF as well as EAX. There are no equivalent instructions for other condition flags, just DF (string direction) and IF (interrupts). The carry flag is special for a lot of instructions; shifts set it, adc al, 0 can add it to AL in 2 byte, and I mentioned earlier the undocumented SALC.

std / cld rarely seem worth it. Especially in 32-bit code, it's better to just use dec on a pointer and a mov or memory source operand to an ALU instruction instead of setting DF so lodsb / stosb go downward instead of up. Usually if you need downward at all, you still have another pointer going up, so you'd need more than one std and cld in the whole function to use lods / stos for both. Instead, just use the string instructions for the upward direction. (The standard calling conventions guarantee DF=0 on function entry, so you can assume that for free without using cld.)

### 8086 history: why these encodings exist

In original 8086, AX was very special: instructions like lodsb / stosb, cbw, mul / div and others use it implicitly. That's still the case of course; current x86 hasn't dropped any of 8086's opcodes (at least not any of the officially documented ones, except in 64-bit mode). But later CPUs added new instructions that gave better / more efficient ways to do things without copying or swapping them to AX first. (Or to EAX in 32-bit mode.)

e.g. 8086 lacked later additions like movsx / movzx to load or move + sign-extend, or 2 and 3-operand imul cx, bx, 1234 that don't produce a high-half result and don't have any implicit operands.

Also, 8086's main bottleneck was instruction-fetch, so optimizing for code-size was important for performance back then. 8086's ISA designer (Stephen Morse) spent a lot of opcode coding space on special cases for AX / AL, including special (E)AX/AL-destination opcodes for all the basic immediate-src ALU- instructions, just opcode + immediate with no ModR/M byte. 2-byte add/sub/and/or/xor/cmp/test/... AL,imm8 or AX,imm16 or (in 32-bit mode) EAX,imm32.

But there's no special case for EAX,imm8, so the regular ModR/M encoding of add eax,4 is shorter.

The assumption is that if you're going to work on some data, you'll want it in AX / AL, so swapping a register with AX was something you might want to do, maybe even more often than copying a register to AX with mov.

Everything about 8086 instruction encoding supports this paradigm, from instructions like lodsb/w to all the special-case encodings for immediates with EAX to its implicit use even for multiply/divide.

Don't get carried away; it's not automatically a win to swap everything to EAX, especially if you need to use immediates with 32-bit registers instead of 8-bit. Or if you need to interleave operations on multiple variables in registers at once. Or if you're using instructions with 2 registers, not immediates at all.

But always keep in mind: am I doing anything that would be shorter in EAX/AL? Can I rearrange so I have this in AL, or am I currently taking better advantage of AL with what I'm already using it for.

Mix 8-bit and 32-bit operations freely to take advantage whenever it's safe to do so (you don't need carry-out into the full register or whatever).

• cdq is useful for div which needs zeroed edx in many cases.
– qwr
Apr 14, 2018 at 16:50
• @qwr: right, you can abuse cdq before unsigned div if you know your dividend is below 2^31 (i.e. non-negative when treated as signed), or if you use it before setting eax to a potentially-large value. Normally (outside code-golf) you'd use cdq as setup for idiv, and xor edx,edx before div Apr 14, 2018 at 21:17
• "I've also never found a use for AAA / DAA or other packed-BCD or 2-ASCII-digit instructions." Here's an example of converting a number to an ASCII hex digit codepoint. I golfed this at some point and found several choices, none of which were shorter than this sequence.
– ecm
Sep 7, 2021 at 15:03
• @ecm: There's a 5-byte / 3-instruction hack using DAS (which I didn't know about when I wrote this answer), suggested by @ peter ferrie. I described how/why it works in Little Endian Number to String Conversion Sep 7, 2021 at 18:47
• @ecm: that's correct; 64-bit mode cleaned up the opcode coding space a bit for future 64-bit-only extensions, which Intel has unfortunately been reluctant to take advantage of. Still wasting code-size cramming things like EVEX prefixes into patterns that aren't valid 32-bit encodings. What I said wasn't wrong, since modern CPUs are still required to support 16 and 32-bit modes, but it is useful to clarify, thanks. For actual codegolf.SE purposes, it's sufficient that 64-bit lahf/sahf are available on some implementations, and fuz's linked answer mentions that along with the BCD insns. Feb 8, 2022 at 16:25

### Create 3 zeroes with mul (then inc/dec to get +1 / -1 as well as zero)

You can zero eax and edx by multiplying by zero in a third register.

xor   ebx, ebx      ; 2B  ebx = 0
mul   ebx           ; 2B  eax=edx = 0

inc   ebx           ; 1B  ebx=1


will result in EAX, EDX, and EBX all being zero in just four bytes. You can zero EAX and EDX in three bytes:

xor eax, eax
cdq


But from that starting point you can't get a 3rd zeroed register in one more byte, or a +1 or -1 register in another 2 bytes. Instead, use the mul technique.

Example use-case: concatenating the Fibonacci numbers in binary.

Note that after a LOOP loop finishes, ECX will be zero and can be used to zero EDX and EAX; you don't always have to create the first zero with xor.

• This is a bit confusing. Could you expand? Nov 14, 2017 at 1:29
• @NoOneIsHere I believe he wants to set three registers to 0, including EAX and EDX.
– Maya
Nov 27, 2017 at 15:11
• Another use-case: 8086 Segment Address to Linear using 32-bit registers in 16-bit code, getting upper-16 zeroed in multiple registers is useful for later zero-extending, and this trick is more likely to be worth it since you're paying two 66h prefixes for three 32-bit registers. And if you don't need all those zeros right away, sometimes it can still allow other savings like mov cl, 8 instead of mov cx, 8 after zeroing ECX as part of this. Dec 24, 2022 at 7:39

0100 81C38000      ADD     BX,0080
0104 83EB80        SUB     BX,-80


• This also works the other direction, of course: add -128 instead of sub 128. Fun fact: compilers know this optimization, and also do a related optimization of turning < 128 into <= 127 to reduce the magnitude of an immediate operand for cmp, or gcc always prefers rearranging compares to reduce the magnitude even if it's not -129 vs. -128. May 18, 2018 at 20:22

# Skipping instructions

Skipping instructions are opcode fragments that combine with one or more subsequent opcodes. The subsequent opcodes can be used with a different entrypoint than the prepended skipping instruction. Using a skipping instruction instead of an unconditional short jump can save code space, be faster, and set up incidental state such as NC (No Carry).

My examples are all for 16-bit Real/Virtual 86 Mode, but a lot of these techniques can be used similarly in 16-bit Protected Mode, or 32- or 64-bit modes.

Quoting from my ACEGALS guide:

## 11: Skipping instructions

The constants __TEST_IMM8, __TEST_IMM16, and __TEST_OFS16_IMM8 are defined to the respective byte strings for these instructions. They can be used to skip subsequent instructions that fit into the following 1, 2, or 3 bytes. However, note that they modify the flags register, including always setting NC. The 16-bit offset plus 16-bit immediate test instruction is not included for these purposes because it might access a word at offset 0FFFFh in a segment. Also, the __TEST_OFS16_IMM8 as provided should only be used in 86M, to avoid accessing data beyond a segment limit. After the db instruction using one of these constants, a parenthetical remark should list which instructions are skipped.

The 86 Mode defines in lmacros1.mac 323cc150061e (2021-08-29 21:45:54 +0200):

%define __TEST_IMM8 0A8h                        ; changes flags, NC
%define __TEST_IMM16 0A9h                       ; changes flags, NC
; Longer NOPs require two bytes, like a short jump does.
; However they execute faster than unconditional jumps.
; This one reads random data in the stack segment.
;  (Search for better ones.)
%define __TEST_OFS16_IMM8 0F6h,86h              ; changes flags, NC


The 0F6h,86h opcode in 16-bit modes is a test byte [bp + disp16], imm8 instruction. I believe I am not using this one anywhere actually. (A stack memory access might actually be slower than an unconditional short jump, in fact.)

0A8h is the opcode for test al, imm8 in any mode. The 0A9h opcode changes to an instruction of the form test eax, imm32 in 32- and 64-bit modes.

First, chain two different entrypoints for a common function which both need to initialise a byte-sized register. The mov al, X instructions take 2 bytes each, so __TEST_IMM16 can be used to skip one such instruction. (This pattern can be repeated if there are more than two entrypoints.)

error_fsiboot:
mov al,'I'

db __TEST_IMM16 ; (skip mov)
mov al, 'R'     ; Disk 'R'ead error

error:


Second, a certain entrypoint that needs two bytes worth of additional teardown but can otherwise be shared with the fallthrough case of a later code part.

                mov bx, [VAR(para_per_sector)]
sub word [VAR(paras_left)], bx
jbe @F          ; read enough -->

loop @BB
pop bx
pop cx

call clust_next
inc ax
inc ax
test al, 8      ; set in 0FFF_FFF8h--0FFF_FFFFh,
;  clear in 0, 1, and 0FFF_FFF7h
db __TEST_IMM16
@@:
pop bx
pop cx
call check_enough
jmp near word [VAR(fsiboot_table.success)]


Here's a use case in inicomp lz4.asm 4d568330924c (2021-09-03 16:59:42 +0200) where we depend on the test al, X instruction clearing the Carry Flag:

.success:
db __TEST_IMM8                  ; (NC)
.error:
stc
retn


Finally, here's a very similar use of a skipping instruction in DOSLFN Version 0.41c (11/2012). Instead of test ax, imm16 they're using mov cx, imm16 which has no effect on the status flags but clobbers the cx register instead. (Opcode 0B9h is mov ecx, imm32 in non-16-bit modes, and writes to the full ecx or rcx register.)

;THROW-Geschichten... [english: THROW stories...]
SetErr18:
mov     al,18
db      0B9h            ;mov cx,nnnn
SetErr5:
mov     al,5
db      0B9h            ;mov cx,nnnn
SetErr3:
mov     al,3
db      0B9h            ;mov cx,nnnn
SetErr2:
mov     al,2
SetError:

• Welcome to Code Golf! This is a great tip! Sep 20, 2021 at 13:27
• My answer on Golf a Custom Fibonacci Sequence is another example of the same idea. The first instruction I wanted for my loop happened to be opcode 01 add, so an EB byte before the loop made the first trip through decode as jmp rel8=+1, jumping to the end of the 2-byte add instruction. I first learned of the idea from Ira Baxter's answer on Can assembled ASM code result in more than a single possible way (except for offset values)? Oct 27, 2021 at 17:32

# CPU registers and flags are in known startup states

For a full/standalone program, we can assume that the CPU is in a known and documented default state based on platform and OS.

For example:

Linux x86 ELF http://asm.sourceforge.net/articles/startup.html - in _start in a static executable, most registers are zero other than the stack pointer, to avoid leaking info into a fresh process. pop will load argc which is a small non-negative integer, 1 if run normally from a shell with no args.

Same applies for x86-64 processes on Linux.

• Code Golf rules say your code has to work on at least one implementation. Linux chooses to zero all the regs (except RSP) and stack before entering a fresh user-space process, even though the i386 and x86-64 System V ABI docs say they're "undefined" on entry to _start. So yes it's fair game to take advantage of that if you're writing a program instead of a function. I did so in Extreme Fibonacci. (In a dynamically-linked executable, ld.so runs before jumping to your _start, and does leave garbage in registers, but static is just your code.) May 3, 2019 at 1:50
• A couple others: eflags is set to 0x202, mxcsr is set to 0x1f80, though you can't directly access them. Jan 10, 2020 at 5:04

# mov small immediates into lower registers when applicable

If you already know the upper bits of a register are 0, you can use a shorter instruction to move an immediate into the lower registers.

b8 0a 00 00 00          mov    $0xa,%eax  versus b0 0a mov$0xa,%al


# Use push/pop for imm8 to zero upper bits

Credit to Peter Cordes. xor/mov is 4 bytes, but push/pop is only 3!

6a 0a                   push   $0xa 58 pop %eax  • mov al, 0xa is good if you don't need it zero-extended to the full reg. But if you do, xor/mov is 4 bytes vs. 3 for push imm8/pop or lea from another known constant. This could be useful in combination with mul to zero 3 registers in 4 bytes, or cdq, if you need a lot of constants, though. Mar 29, 2018 at 18:13 • The other use-case would be for constants from [0x80..0xFF], which are not representable as a sign-extended imm8. Or if you already know the upper bytes, e.g. mov cl, 0x10 after a loop instruction, because the only way for loop to not jump is when it made rcx=0. (I guess you said this, but your example uses an xor). You can even use the low byte of a register for something else, as long as the something else puts it back to zero (or whatever) when you're done. e.g. my Fibonacci program keeps -1024 in ebx, and uses bl. Mar 29, 2018 at 18:17 • @PeterCordes I've added your push/pop technique – qwr Mar 29, 2018 at 18:30 • Should probably go into the existing answer about constants, where anatolyg already suggested it in a comment. I'll edit that answer. IMO you should rework this one to suggest using 8-bit operand-size for more stuff (except xchg eax, r32) e.g. mov bl, 10 / dec bl / jnz so your code doesn't care about the high bytes of RBX. Mar 29, 2018 at 18:37 • Caveat is that PUSH immediate is not supported on the 8086/8088. Feb 18, 2019 at 19:16 # The FLAGS are set after many instructions After many arithmetic instructions, the Carry Flag (unsigned) and Overflow Flag (signed) are set automatically (more info). The Sign Flag and Zero Flag are set after many arithmetic and logical operations. This can be used for conditional branching. Example: d1 f8 sar %eax  ZF is set by this instruction, so we can use it for condtional branching. • When have you ever used the parity flag? You know it's the horizontal xor of the low 8 bits of the result, right? (Regardless of operand-size, PF is set only from the low 8 bits; see also). Not even-number / odd-number; for that check ZF after test al,1; you usually don't get that for free. (Or and al,1 to create an integer 0/1 depending on odd/even.) Mar 29, 2018 at 18:46 • Anyway, if this answer said "use flags already set by other instructions to avoid test / cmp", then that would be pretty basic beginner x86, but still worth an upvote. Mar 29, 2018 at 18:49 • @PeterCordes Huh, I seemed to have misunderstood the parity flag. I am still working on my other answer. I'll edit the answer. And as you can probably tell, I am a beginner so basic tips help. – qwr Mar 29, 2018 at 18:55 # lea for math This is probably one of the first things one learns about x86, but I leave it here as a reminder. lea can be used to do multiplication by 2, 3, 4, 5, 8, or 9, and adding an offset. For example, to calculate ebx = 9*eax + 3 in one instruction (in 32-bit mode): 8d 5c c0 03 lea 0x3(%eax,%eax,8),%ebx  Here it is without an offset: 8d 1c c0 lea (%eax,%eax,8),%ebx  Wow! Of course, lea can be used to also do math like ebx = edx + 8*eax + 3 for calculating array indexing. • Maybe worth mentioning that lea eax, [rcx + 13] is the no-extra-prefixes version for 64-bit mode. 32-bit operand-size (for the result) and 64-bit address size (for the inputs). Mar 30, 2018 at 17:39 # Use do-while loops instead of while loops This is not x86 specific but is a widely applicable beginner assembly tip. If you know a while loop will run at least once, rewriting the loop as a do-while loop, with loop condition checking at the end, often saves a 2 byte jump instruction. In a special case you might even be able to use loop. • Related: Why are loops always compiled like this? explains why do{}while() is the natural looping idiom in assembly (especially for efficiency). Note also that 2-byte jecxz/jrcxz before a loop works very well with loop to handle the "needs to run zero times" case "efficiently" (on the rare CPUs where loop isn't slow). jecxz is also usable inside the loop to implement a while(ecx){}, with jmp at the bottom. Apr 15, 2018 at 0:02 • @PeterCordes that is a very well written answer. I'd like to find a use for jumping into the middle of a loop in a code golf program. – qwr Apr 15, 2018 at 2:56 • Use goto jmp and indentation... Loop follow – user58988 Mar 12, 2019 at 19:04 # Use fastcall conventions x86 platform has many calling conventions. You should use those that pass parameters in registers. On x86_64, the first few parameters are passed in registers anyway, so no problem there. On 32-bit platforms, the default calling convention (cdecl) passes parameters in stack, which is no good for golfing - accessing parameters on stack requires long instructions. When using fastcall on 32-bit platforms, 2 first parameters are usually passed in ecx and edx. If your function has 3 parameters, you might consider implementing it on a 64-bit platform. C function prototypes for fastcall convention (taken from this example answer): extern int __fastcall SwapParity(int value); // MSVC extern int __attribute__((fastcall)) SwapParity(int value); // GNU  Note: you can also use other calling conventions, including custom ones. I never use custom calling conventions; for any ideas related to these, see here. • Or use a fully custom calling convention, because you're writing in pure asm, not necessarily writing code to be called from C. Returning booleans in FLAGS is often convenient. Aug 4, 2018 at 21:32 # Combinations with CDQ for certain piecewise-linear functions CDQ sign-extends EAX into EDX, making EDX 0 if EAX is nonnegative and -1 (all 1s) if EAX is negative. This can be combined with several other instructions to apply certain piecewise-linear functions to a value in EAX in 3 bytes: CDQ + AND$$\ \min(x, 0) \$$ (in either EAX or EDX). (I have used this here.) CDQ + OR$$\ \max(x, -1) \$$. CDQ + XOR$$\ \max(x, -x-1) \$$. CDQ + MUL EDX$$\ \max(-x, 0) \$$ in EAX and $$\ \left\{ \begin{array}{ll} 0 & : x \ge 0 \\ x - 1 & : x < 0 \end{array} \right.\$$ in EDX. The loop and string instructions are smaller than alternative instruction sequences. Most useful is loop <label> which is smaller than the two instruction sequence dec ECX and jnz <label>, and lodsb is smaller than mov al,[esi] and inc si. # Entry point doesn't necessarily have to be first byte of submission I came across this answer, and didn't understand it at first until I realized that the intention is: ; ---- example calling code starts here ------------- MOV ECX, 1 CALL entry RET ; ---- code golf answer code starts here (5 bytes) -- 41 INC ECX entry: E3 FD JECXZ SHORT$-1
91       XCHG    EAX,ECX
C3       RETN

; ---- code golf answer code ends here -------------


Does not seem to conflict with any of the conditions of "Choose your calling convention" and is otherwise valid assembly language.

• I see nothing wrong with that. The reason you don't see it in other languages is because they don't allow you to. Assembly is just bytes. Jan 27, 2021 at 23:50
• You can do this in real programs using real asm / C toolchains, so it seems perfectly fine to me. From the PoV of anything else, you could look at this as the function jumping to a helper function next to it, if you're using tools that insist on treating the bytes of the function proper as the ones following the label. Does a function with instructions before the entry-point cause problems for anything? - AFAIK it's fine even in Linux shared or static libraries (maybe requiring labels and separate .size metadata for the part before the function entry) Apr 24, 2021 at 6:58

# Use a good assembler

There are dozens of x86 assemblers out there, and they are not created equal.

Not only can a bad assembler be painful to use, but they might not always output the most optimal code.

Most x86 instructions have multiple valid encodings, some shorter than others.

For example, I saw one user with a 16-bit assembler that emitted different code depending on the order of xchg's operands. It is a commutative operation, it shouldn't make a difference.

87 D8   xchg    ax, bx
93      xchg    bx, ax


Life is too short for bad assemblers, and it should not be the thing getting in the way of golfing.

The three assemblers I would suggest are:

• nasm is the first one I would recommend to everyone (and I wish I had learned it first).
• It fully supports 16-bit, 32-bit, and 64-bit code
• It can easily assemble all sorts of object formats, as well as raw binaries/.com files (a multi-step ritual with GAS).
• It is officially supported on DOS as well as all modern OSes.
• While it doesn't support C macros, it has a god-tier preprocessor that is much better than C.
• The way it handles local labels is really nice.
• Good error messages, mostly fairly beginner-friendly pointing you in the direction of why an instruction isn't allowed or what it doesn't like about a source line.
• GAS (GCC's assembler) is another fairly good assembler.
• It is the assembler used by GCC and Clang. You might recognize it if you use Godbolt or gcc -S.
• It supports AT&T syntax which some might prefer
• With .intel_syntax noprefix, you can switch to Intel syntax
• While its built-in preprocessor is pretty limited, it can be easily combined with a C preprocessor.
• For x87, beware of the AT&T syntax design bug that interchanges fsubr with fsub in some cases, same for fdiv[r]. Older GAS versions applied the same swap in Intel-syntax mode, and so did older binutils objdump -d versions. (This is AT&T's fault, not GNU's, and current GAS versions do as well as possible, but is an inherent downside in using AT&T syntax for x87.)
• Error messages are less helpful than NASM about why an instruction is invalid
• Ambiguous instructions other than mov default to dword operand-size instead of being an error in AT&T syntax, such as add $123, (%rdi) assembling as addl. Clang, and GAS in Intel syntax mode, error on this. • Clang is, in most cases, completely exchangeable for GAS. • It has much more helpful error messages and doesn't silently treat x86_64 registers as symbols in 32-bit mode. • While AT&T syntax is fully supported, Intel syntax currently has a few bugs. • It is the only reason I am recommending GAS over Clang. 😔 • It still works as a solid linter. • It also supports C macros. I haven't used enough of the other assemblers to give a good opinion, as I am more than satisfied with those three. FASM is also well-regarded, using very nearly the same syntax as NASM, and is supported on https://TIO.run/ ; It's able to make 32-bit executables on TIO, unlike with other assemblers, using the directive format ELF executable 3 to emit a 32-bit ELF executable, not a .o object file that would need linking. EuroAssembler is also open-source; its maintainer is active on Stack Overflow in the [assembly] and [x86] tags. To add or subtract 1, use the one byte inc or dec instructions which are smaller than the multibyte add and sub instructions. • Note that 32-bit mode has 1-byte inc/dec r32 with the register number encoded in the opcode. So inc ebx is 1 byte, but inc bl is 2. Still smaller than add bl, 1 of course, for registers other than al. Also note that inc/dec leave CF unmodified, but update the other flags. Mar 29, 2018 at 18:56 • 2 for +2 & -2 in x86 – l4m2 Mar 31, 2018 at 4:39 # Try XLAT for byte memory access XLAT is a one byte instruction that is equivalent to AL = [BX+AL]. Yes, that's right, it lets you use AL as an index register for memory access. • Has this ever been used for golfing? I remember that I wanted to use it several times, but each time I found a shorter way to do the same. May 20, 2020 at 13:38 • @anatolyg It has limited use for sure, but I used it here and here and probably saved a byte or two. The latter being more the canonical usage. May 20, 2020 at 13:47 • @anatolyg Combining with other short opcodes that operate on AL like LODSB, AAM, AAD, INT 10/21H, it could be golfy. For example, this code char bx[] = "qwertyuiop"; al = bx[ al % 10 ]; "get an arbitary character in a string using the rightmost digit in accumulator as index" would be 3 bytes AAM / XLAT. Used here. May 20, 2020 at 14:09 # Take advantage of the x86_64 code model Linux's default code model will put all of your code and globals in the low 31 bits of memory, so 32-bit pointer arithmetic here is perfectly safe. The stack, libraries, and any dynamically allocated pointers are not, though. Try it online! Make sure to still use the entire 64 bits in memory operands (including lea), because using [eax] requires a 67 prefix byte. • The Linux x32 ABI (ILP32 in 64-bit mode) puts all addresses, including stack, in the low 32 bits of virtual address space. You can say you're targeting that if you want to add edi, 4 instead of add rdi,4. (Although in that specific example, scasd will increment RDI by 4, assuming you haven't changed DF and it doesn't segfault). You can copy a 64-bit pointer in 2 bytes with push/pop, and maybe get away with only comparing the low 32 bits (of pointers to the same array), so there's a lot you can do while still being mostly 64-bit clean. Oct 27, 2021 at 17:27 # Use interrupts and syscalls wisely In general, unlike the C calling conventions, most syscalls and interrupts will preserve your registers and flags unless noted otherwise, except for a return value usually in AL/EAX/RAX depending on the OS. (e.g. the x86-64 syscall instruction itself destroys RCX and R11) #### Linux specific: • If exiting with a crash is okay, int3, int1, or into can usually do the job in one byte. Don't try this on DOS though, it will lock up. • Note that the error messages like Segmentation fault (core dumped) or Trace/breakpoint trap are actually printed by your shell, not the program/kernel. Don't believe me? Try running set +m before your program or redirecting stderr to a file. • You can use int 0x80 in 64-bit mode. It will use the 32-bit ABI though (eax, ebx, ecx, edx), so make sure all pointers are in the low 32 bits. On the small code model, this true for all code stored in your binary. Keep this in mind for . • Additionally, sysenter and call dword gs:0x10 can also do syscalls in 32-bit mode, although the calling convention is quite....weird for the former. #### DOS/BIOS specific: • Use int 29h instead of int 21h:02h for printing single bytes to the screen. int 29h doesn't need ah to be set and very conveniently uses al instead of dl. It writes directly to the screen, so you can't just redirect to a file, though. • DOS also has strlen and strcmp interrupts (see this helpful page for this and other undocumented goodies) • Unless you modified cs, don't use int 20h or int 21h:4Ch for exiting, just ret from your .com file. Alternatively, if you happen to have 0x0000 on the top of your stack, you can also ret to that. • In the rare case that you need to call helper functions more than 4 times, consider registering them to the int1, int3, or into interrupts. Make sure to use iret instead of ret, though.  ; AH: 0x25 (set interrupt) ; AL: 0x03 (INT3) ; Use 0x2504 for INTO, or 0x2501 for INT1 mov ax, 0x2503 ; DX: address to function (make sure to use IRET instead of RET) mov dx, my_interrupt_func int 0x21 ; Now instead of this 3 byte opcode... call my_func ; ...just do this 1 byte opcode. int3 ; or int1, into  • tip: Your interrupt vector table is a table of far pointers at 0000:0000 (so for example, int 21h is at 0000h:0084h). • re: repeated calls: Shorter x86 call instruction - getting a function-pointer into a register only takes 3 or 5 bytes (16 vs. 32/64-bit modes), and allows 2-byte call reg. In 32-bit mode, this comes out ahead for just two call-sites. (5 + 2x2 vs. 5x2). That does cost a register, unlike int3. Aug 24, 2021 at 16:59 • Spending 8 bytes to set up an int3 handler in real mode breaks even if you amortize over 4 calls. (3*n = 3+3+2 + 1*n solves to n = 4.) mov reg,func / call reg in 16-bit mode amortizes to break-even + costing a register over 3 calls (3*n = 3 + 2*n). It loses to int3 at 6 or more calls (assuming you can spare a register), break even at 5 (3 + 2*n = 3+3+2 + 1*n) Aug 24, 2021 at 17:00 # Use conditional moves CMOVcc and sets SETcc This is more a reminder to myself, but conditional set instructions exist and conditional move instructions exist on processors P6 (Pentium Pro) or newer. There are many instructions that are based on one or more of the flags set in EFLAGS. • I've found branching is usually smaller. There are some cases where it's a natural fit, but cmov has a 2-byte opcode (0F 4x +ModR/M) so it's 3 bytes minimum. But the source is r/m32, so you can conditionally load in 3 bytes. Other than branching, setcc is useful in more cases than cmovcc. Still, consider the entire instruction set, not just baseline 386 instructions. (Although SSE2 and BMI/BMI2 instruction are so large that they're rarely useful. rorx eax, ecx, 32 is 6 bytes, longer than mov + ror. Nice for performance, not golf unless POPCNT or PDEP saves many isns) Mar 29, 2018 at 18:33 • @PeterCordes thanks, I've added setcc. – qwr Mar 29, 2018 at 18:36 • (BTW, "conditionally load" isn't quite accurate in my last comment. Memory-source cmov is an unconditional load, which will fault on a bad address. And then an ALU select. Not like an ARM predicated ldreq or whatever.) Dec 24, 2022 at 7:32 # Use whatever calling conventions are convenient System V x86 uses the stack and System V x86-64 uses rdi, rsi, rdx, rcx, etc. for input parameters, and rax as the return value, but it is perfectly reasonable to use your own calling convention. __fastcall uses ecx and edx as input parameters, and other compilers/OSes use their own conventions. Use the stack and whatever registers as input/output when convenient. Example: The repetitive byte counter, using a clever calling convention for a 1 byte solution. Other resources: Agner Fog's notes on calling conventions • I finally got around to posting my own answer on this question about making up calling conventions, and what's reasonable vs unreasonable. May 18, 2018 at 5:05 • @PeterCordes unrelated, what is the best way to print in x86? So far I've avoided challenges that require printing. DOS looks like it has useful interrupts for I/O but I am only planning on writing 32/64 bit answers. The only way I know of is int 0x80 which requires a bunch of setup. – qwr May 18, 2018 at 19:23 • Yeah, int 0x80 in 32-bit code, or syscall in 64-bit code, to invoke sys_write, is the only good way. It's what I used for Extreme Fibonacci. In 64-bit code, __NR_write = 1 = STDOUT_FILENO, so you can mov eax, edi. Or if the upper bytes of EAX are zero, mov al, 4 in 32-bit code. You could also call printf or puts, I guess, and write a "x86 asm for Linux+glibc" answer. I think it's reasonable to not count the PLT or GOT entry space, or the library code itself. May 18, 2018 at 20:02 • I'd be more inclined to have the caller pass a char*buf and produce the string in that, with manual formatting. e.g. like this (awkwardly optimized for speed) asm FizzBuzz, where I got string data into register and then stored it with mov, because the strings were short and fixed-length. May 18, 2018 at 20:04 # Save on jmp bytes by arranging into if/then rather than if/then/else This is certainly very basic, just thought I would post this as something to think about when golfing. As an example, consider the following straightforward code to decode a hexadecimal digit character:  cmp$'A', %al
jae .Lletter
sub $'0', %al jmp .Lprocess .Lletter: sub$('A'-10), %al
.Lprocess:
movzbl %al, %eax
...


This can be shortened by two bytes by letting a "then" case fall into an "else" case:

    cmp $'A', %al jb .digit sub$('A'-'0'-10), %eax
.digit:
sub $'0', %eax movzbl %al, %eax ...  • You'd often do this normally when optimizing for performance, especially when the extra sub latency on the critical path for one case isn't part of a loop-carried dependency chain (like here where each input digit is independent until merging 4-bit chunks). But I guess +1 anyway. BTW, your example has a separate missed optimization: if you're going to need a movzx at the end anyway then use sub$imm, %al not EAX to take advantage of the no-modrm 2-byte encoding of op $imm, %al. Aug 24, 2019 at 5:05 • Also, you can eliminate the cmp by doing sub$'A'-10, %al ; jae .was_alpha ; add $('A'-10)-'0'. (I think I got the logic right). Note that 'A'-10 > '9' so there's no ambiguity. Subtracting the correction for a letter will wrap a decimal digit. So this is safe if we're assuming our input is valid hex, just like yours does. Aug 24, 2019 at 5:10 # (way too many) ways of zeroing a register I remember being taught these by a certain person (I "invented" some of these myself); I don't remember who did I get them from, anyways these are the most interesting; possible use cases include restricted source code challenges or other bizzare stuff. => Zero mov: mov reg, 0 ; mov eax, 0: B800000000  => push+pop: push [something equal to zero] pop reg ; push 0 / pop eax: 6A0058 ; note: if you have a register equal to zero, it will be ; shorter but also equal to a mov.  => sub from itself: sub reg, reg ; sub eax, eax: 29C0  => mul by zero: imul reg, 0 ; imul eax, 0: 6BC000  => and by zero: and reg, 0 ; and eax, 0: 83E000  => xor by itself: xor reg, reg ; xor eax, eax: 31C0 ; possibly the best way to zero an arbitrary register, ; I remembered this opcode (among other).  => or and inc / not: or reg, -1 inc reg ; or not reg ; or eax, -1 / inc eax: 83C8FF40  => reset ECX: loop$

; loop $: E2FE  => flush EDX: shr eax, 1 cdq ; D1E899  => zero AL (AH = AL, AL = 0) aam 1 ; D401  => reset AH: aad 0 ; D500  => Read 0 from the port mov dx, 81h in al, dx ; 66BA8100EC  => Reset AL stc setnc al ; F90F93C0  => Use the zero descriptor from gdt: sgdt [esp-6] mov reg, [esp-4] mov reg, [reg] ; with eax: 0F014424FA8B4424FC8B00  => Read zero from the fs segment (PE exe only) mov reg, fs:[10h] ; with eax: 64A110000000  => The brainfuck way inc reg jnz$-1
; with eax: 4075FD


=> Utilize the coprocessor

fldz
fistp dword ptr [esp-4]
mov eax, [esp-4]
; D9EEDB5C24FC8B4424FC


Another possible options:

• Read zero using the builtin random number generator.
• calculate sine from pi * n (use fmul).

There are way cooler and potentially useful ways to execute this operation; although I didn't come up with them, therefore I'm not posting.

• If you really just want a zero, most of these are uselessly large >.<. This answer might be a better fit for this SO question: How many ways to set a register to zero?. My answer there has quite a lot of 1-instruction ways: if we allow 2 or more instructions, the possibilities become nearly endless. Some of these are interesting, though, like AAD 0 or AAM 1. (Your loop \$ should probably go next to your inc / jnz loop; they're nearly the same thing.) And BTW, yes xor-zeroing is the most efficient choice. Jan 24, 2021 at 19:17

# Use multiplication for hashing

IMUL, multiplication by an immediate signed number, is a powerful instruction which can be used for hashing.

The regular multiplication instruction hard-codes one of the input operands and the output operand to be in eax (or ax or al). This is inconvenient; it requires instructions for setup and sometimes also to save and restore eax and edx. But if one of the operands is a constant, the instruction becomes much more versatile:

• No need to load the constant into a register
• The other operand can be in any register, not only eax
• The result can be in any register, not necessarily overwriting the input!
• The result is 32-bit, not a pair of registers
• If the constant is between -128 and 127, it can be encoded by only one byte

I used this many times (I hope I can be excused for these shameless plugs: 1 2 3 ...)

"Free bypass": If you already have an instruction with an immediate on a register that you only care about the low part of, making the immediate longer than necessary could allow you to insert into its high part other instructions that can be jumped to (but don't execute when coming from before). This works because of little-endianness. Example; another example.

To copy a 64-bit register, use push rcx ; pop rdx instead of a 3-byte mov.
The default operand-size of push/pop is 64-bit without needing a REX prefix.

  51                      push   rcx
5a                      pop    rdx
vs.
48 89 ca                mov    rdx,rcx


(An operand-size prefix can override the push/pop size to 16-bit, but 32-bit push/pop operand-size is not encodeable in 64-bit mode even with REX.W=0.)

If either or both registers are r8..r15, use mov because push and/or pop will need a REX prefix. Worst case this actually loses if both need REX prefixes. Obviously you should usually avoid r8..r15 anyway in code golf.

You can keep your source more readable while developing with this NASM macro. Just remember that it steps on the 8 bytes below RSP. (In the red-zone in x86-64 System V). But under normal conditions it's a drop-in replacement for 64-bit mov r64,r64 or mov r64, -128..127

    ; mov  %1, %2       ; use this macro to copy 64-bit registers in 2 bytes (no REX prefix)
%macro MOVE 2
push  %2
pop   %1
%endmacro


Examples:

   MOVE  rax, rsi            ; 2 bytes  (push + pop)
MOVE  rbp, rdx            ; 2 bytes  (push + pop)
mov   ecx, edi            ; 2 bytes.  32-bit operand size doesn't need REX prefixes

MOVE  r8, r10             ; 4 bytes, don't use
mov   r8, r10             ; 3 bytes, REX prefix has W=1 and the bits for reg and r/m being high

xchg  eax, edi            ; 1 byte  (special xchg-with-accumulator opcodes)
xchg  rax, rdi            ; 2 bytes (REX.W + that)

xchg  ecx, edx            ; 2 bytes (normal xchg + modrm)
xchg  rcx, rdx            ; 3 bytes (normal REX + xchg + modrm)


The xchg part of the example is because sometimes you need to get a value into EAX or RAX and don't care about preserving the old copy. push/pop doesn't help you actually exchange, though.

# Try AAM or AAD for byte division operations

If you are working with only 8 bit values, using the AAM instruction can sometimes save several bytes over DIV reg8 since it will take an imm8 and returns remainder and quotient in opposite AH/AL registers as DIV.

D4 0A    AAM        ; AH = AL / 10, AL = AL % 10


It can also accept any byte value as the divisor as well by altering the second byte.

D4 XX    AAM  XX    ; AH = AL / XX, AL = AL % XX


And AAD is the inverse of this, which is two operations in one.

D5 XX    AAD  XX    ; AL = AH * XX + AL

• Not longer supported in X86-64 Jan 6, 2021 at 19:16

# Avoid registers which need prefixes

Quite a simple tip I haven't seen mentioned before.

Avoid r8-r15 (as well as dil, sil, bpl, and spl) on x86_64 like the plague. Even just thinking about these registers requires an extra REX prefix. The only exception is if you are using them exclusively for 64-bit arithmetic (which also needs REX prefixes). Even still, you are usually better off using a low register since some operations can be done using the implicit zero extension.

Note that this tip also applies to ARM Thumb-2.

Additionally, be careful when using 16-bit registers in 32/64-bit mode (as well as 32-bit registers in 16-bit mode, but this is rare), as these need a prefix byte as well.

However, unlike the extra x86_64 registers, 16-bit instructions can be useful: Many instructions which would otherwise need a full 32-bit immediate argument will only use a 16-bit argument. So, if you were to bitwise and eax by 0xfffff00f, and ax, 0xf00f would be smaller.