CMP and JGE not working in Turbo C++

Question

I am working on a Mix program in C++ and Assembly Language(8086) to find smallest number from an array. Here is my code

#include<iostream>
#include<conio.h>
using namespace std;
void main()
{
__int16 a[5],x,y,res;
int i,j;
y=999;

cout<<"\n Enter 5 Numbers:";
for(i=0;i<5;i++)
{
    cin>>a[i];
}

_asm{
    mov bx,y
}

//Finding smallest
for(i=0;i<5;i++)
{
    x=a[i];
    _asm{
        mov ax,x
        cmp ax,bx
        jge nxt
        mov bx,ax
        nxt:
    }
}

_asm{
    mov res,bx;
}

cout<<"\n Smallest Element:"<<res;
getch();
}

The above code is written in Visual Studio 2010 and seems to be working fine. But when I update the same code for Turbo c++ (i.e. changing "iostream" to "iostream.h", removing "using namespace std;", changing "__int16" to "int" etc), it does not work. The answer produced after execution is wrong.

Here is my TC++ program for same

#include<iostream.h>
#include<conio.h>
void main()
{
int a[5],x,y,res;
int i,j;
y=999;

cout<<"\n Enter 5 Numbers:";
for(i=0;i<5;i++)
{
    cin>>a[i];
}

_asm{
    mov bx,y
}

//Finding smallest
for(i=0;i<5;i++)
{
    x=a[i];
    _asm{
        mov ax,x
        cmp ax,bx
        jge nxt
        mov bx,ax
    }
    nxt:
}

_asm{
    mov res,bx;
}

cout<<"\n Smallest Element:"<<res;
getch();
}

Why TC++ and Visual Studio 10 do not give same answer?

Answer 1

You can't expect registers to retain their values between assembly snippets. You have three assembly snippets, with chunks of C between them, and they rely on bx staying the same. The compiler makes no such promise.

Either use memory to store the running minimum, or reformulate with a single assembly snippet. For the latter approach, you'd have to rewrite the for loop and the array access in assembly; it's quite doable. Like this:

_asm{
mov dx, y ; we'll use dx instead of bx for the running minimum - long story
mov bx, a   ; that's the array pointer
mov si, 0 ; that's our i
loop:
    mov ax, [bx+si*2] ; read a[i] into ax; *2 because int is two bytes
    cmp ax,dx
    jge nxt
    mov dx, ax
    nxt:
    ;Now the for loop stuff
    inc si ; i++
    cmp si, 5 ; compare i to 5
    jl loop   ; if less, continue looping
; End of loop
mov res,dx;
}

I'm using bx and si for base+index memory access because on early x86 CPUs, you could only do that kind of memory access with a limited subset of registers (bx or bp for base, si or di for index). These days, you can use any combination of registers; but I'm not sure if antique Turbo C would take that.