x86 - (fldcw [sp]) Control Word issue in assembly code -


i new assembly & first programming language.

i have question line:fldcw [sp]. causes build error: error a2031: must index or base register

i aware that:

sp 16 bit stack pointer

esp 32 bit stack pointer

-

i trying learn utilizing fpu control word. use skylake processor.

all information obtained at: http://www.website.masmforum.com/tutorials/fptute/fpuchap3.htm#fstcw

when replace esp sp builds fine.

am misunderstanding guide? causing error?

.386 .model flat, stdcall option casemap :none    includelib \masm32\lib\msvcrt.lib sprintf proto c :vararg includelib \masm32\lib\user32.lib  messageboxa proto :ptr,:ptr,:ptr,:dword includelib \masm32\lib\kernel32.lib exitprocess proto :dword   .data    _title db "result",13,10,0    $interm db "%0.4f","+","%0.5f",13,10,0    oldcw   dw   ?    .code main proc local szbuf[9]:byte       fstcw oldcw     ;get current control word retain setting bits                   ;not related rounding control (rc) bits   fwait       ;to insure storage instruction completed   mov   ax,oldcw ; ,   ax,0f3ffh ;clears rc bits, leaving other bits unchanged                   ;not necessary here because both bits set   or    ax,0c00h  ;this set both bits of rc field truncating mode                   ;without affecting of other field's bits   push  eax       ;use stack store modified control word in memory   fldcw [sp]      ;load modified control word      fldcw oldcw     ;restore previous control word   pop   eax       ;clean-up stack                   ;this retrieve 16-bit or 32-bit integer                   ;possibly returned "other fpu instruction(s)"  finished:        invoke sprintf, addr szbuf, offset $interm, eax, edx    invoke messageboxa, 0, addr szbuf, offset _title, 0    invoke exitprocess, 0    main endp end main

in 16 bit mode, [sp] not valid memory operand. following memory operands valid, each having optional displacement:

[bx] [bx+si] [bx+di] [bp] [bp+si] [bp+di] [si] [di] [addr16]

to fix code, recommend set stack frame , use bp-relative addressing:

push bp       ; establish stack frame mov bp,sp     ; dito fstcw oldcw fwait mov ax,oldcw or ax,0c00h push ax fldcw [bp-2]  ; load control word stack leave         ; tear down stack frame

however, if in 32 bit mode, should refer esp, 32 bit stack pointer instead. don't ever use 16 bit registers in memory operands unless in 16 bit mode or know doing:

fstcw oldcw fwait mov ax,oldcw or ax,0c00h push eax fldcw [esp]   ; load control word stack pop eax       ; restore stack

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