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The golfed version actually changed substantially compared to the ungolfed version here, under the (welcome) pressure of Oberon's fine answer.

The golfed version actually changed substantially compared to the ungolfed version here, under the (welcome) pressure of Oberon's fine answer.

Ungolfed:

The golfed version actually changed substantially compared to the ungolfed version here, under the (welcome) pressure of Oberon's fine answer.

I found that replacing the switch statement in favor of an if...else chain enabled me to factor-out all of the digits from my cases. Instead it initializes the w variable to 47 so one increment raises it to 48 (== ascii '0') then each case increments w until we need to skip to 'A' at which point we make use of the mostly empty first macro argument which adds an extra 7 to get up to 'A'. The ungolfed version does show my favorite sbrk/SIGSEGV trick to get "free" memory with no further allocations.

#include<math.h>
#include<signal.h>
void S(int x){signal(SIGSEGV,S);sbrk(8*8*8);}
int*s,*t,*i,w,a,c,d;    //stack top index working accumulator count data
u(x){*t++=x;}           //push()
o(){return t>s?*--t:0;} //pop()
p(x){                   //print()
    if(x<0){    //negative?
        W(*"-") //output '-'
        x=-x;   //negate
    }
    if(x>9)     //more than one digit?
        p(x/10); //recurse after integer-divide
    b=48+x%10;   //isolate and convert single digit to ascii
    W(b)         //output ascii digit
}
main(){
    char b[1];
    signal(SIGSEGV,S);  //auto-allocate memory for stack
    t=s=sbrk(8*8*8);  //get start of memory and allocate
    while(read(0,b,1)){
        write(1,b,1); //for debugging: echo the command being executed
        switch(*b){
            case '0': w=o(); a=o(); for(c=ceil(log10(w));c>0;c--) a*=10; u(a+w); break;
            case '1': u(o()+1); break;
            case '2': u(o()-1); break;
            case '3': w=o(); u(o()*w); break;
            case '4': w=o(); u(o()/w); break;
            case '5': u(o()+o()); break;
            case '6': w=o(); u(o()-w); break;
            case '7': c=o();a=1; for(w=o();c>0;c--) a*=w; u(a); break;
            case '8': w=o(); u(o()%w); break;
            case '9': w=*s; memmove(s,s+1,4*(t-s-1)); t[-1]=w; break;
            case 'A': w=t[-1]; memmove(s+1,s,4*(t-s-1)); *s=w; break;
            case 'B': w=o(); u(w); u(w); break;
            case 'C': w=o(); a=o(); u(a); u(w); u(a); u(w); break;
            case 'D': w=o(); a=o(); u(w); u(a); break;
            case 'E': w=o(); a=o(); c=o(); d=o(); u(a); u(w); u(d); u(c); break;
            case 'F': o(); break;
        }
    }
    write(1,"\n[",2);   //dump the stack
    i=t;
    do {
        p(*--i);
    } while(i>s && write(1,",",1));
    write(1,"]\n",2);
}

#include<math.h>
#include<signal.h>
void S(int x){signal(SIGSEGV,S);sbrk(8*8*8);}
int*s,*t,*i,w,a,c,d;    //stack top index working accumulator count data
u(x){*t++=x;}           //push()
o(){return t>s?*--t:0;} //pop()
#define W(x)write(1,&x,1);  //output a byte
p(x){                   //print()
    if(x<0){    //negative?
        W(*"-") //output '-'
        x=-x;   //negate
    }
    if(x>9)     //more than one digit?
        p(x/10); //recurse after integer-divide
    b=48+x%10;   //isolate and convert single digit to ascii
    W(b)         //output ascii digit
}
main(){
    char b[1];
    signal(SIGSEGV,S);  //auto-allocate memory for stack
    t=s=sbrk(8*8*8);  //get start of memory and allocate
    while(read(0,b,1)){
        write(1,b,1); //for debugging: echo the command being executed
        switch(*b){
            case '0': w=o(); a=o(); for(c=ceil(log10(w));c>0;c--) a*=10; u(a+w); break;
            case '1': u(o()+1); break;
            case '2': u(o()-1); break;
            case '3': w=o(); u(o()*w); break;
            case '4': w=o(); u(o()/w); break;
            case '5': u(o()+o()); break;
            case '6': w=o(); u(o()-w); break;
            case '7': c=o();a=1; for(w=o();c>0;c--) a*=w; u(a); break;
            case '8': w=o(); u(o()%w); break;
            case '9': w=*s; memmove(s,s+1,4*(t-s-1)); t[-1]=w; break;
            case 'A': w=t[-1]; memmove(s+1,s,4*(t-s-1)); *s=w; break;
            case 'B': w=o(); u(w); u(w); break;
            case 'C': w=o(); a=o(); u(a); u(w); u(a); u(w); break;
            case 'D': w=o(); a=o(); u(w); u(a); break;
            case 'E': w=o(); a=o(); c=o(); d=o(); u(a); u(w); u(d); u(c); break;
            case 'F': o(); break;
        }
    }
    write(1,"\n[",2);   //dump the stack
    i=t;
    do {
        p(*--i);
    } while(i>s && write(1,",",1));
    write(1,"]\n",2);
}