User:Cerisesky

Testing for 32 bit support on x86

I don't know of any official way to test for 32 bit (>386) support. However, I have experimented with different 386 opcodes to determine support for it. A quick warning that this is definitely undefined behaviour though it has been tested on the 8086 using 86box. The way the code works is by attempting to set ds and fs to the same value, and if it can, then 32 bit is supported since fs is only available on 386+. I have tried other opcodes and methods for determining support, but most of them cause some weird issues or cause the cpu to hang on older machines.

cpu 8086
bits 16

; returns 0 if the processor does not support 32 bit mode.
; returns 1 if it does
; destroys the fs register which should be fine since you aren't
; likely to have used it if you don't know if its even available
test_386:
  push ax
  push bx

  mov ax, ds
  push ds
  
  cpu 386
  pop fs
  mov bx, fs
  cpu 8086
  
  cmp ax, bx
  je .success
  .fail:
    mov ax, 0
    jmp .end
  .success:
    mov ax, 1
  .end:
    pop bx
    pop ax
    ret

Testing for 186 support

Probably not as important but for completeness I also made a test for 186. Works by attempting to push an immediate value to the stack and checking if anything on the stack actually changed. Also undefined behaviour but tested on 8086 in 86box.

cpu 8086
bits 16

; returns 0 if the processor does not support 186 instructions
; returns 1 if it does
test_186:
  push bx
  push cx
  mov bx, sp

  cpu 186
  push 0
  cpu 8086
  
  mov cx, bx
  sub cx, sp
  mov sp, bx
  
  cmp cx, 2
  je .success
  .fail:
    mov ax, 0
    jmp .end
  .success:
    mov ax, 1
  .end:
    pop cx
    pop bx
    ret