Q42982: Altering BP Register in In-Line Assembly Block

Article: Q42982
Product(s): See article
Version(s): 2.00
Operating System(s): MS-DOS
Keyword(s): ENDUSER | inline | mspl13_c
Last Modified: 2-MAY-1989

In Microsoft QuickC Version 2.00, altering BP register in an in-line
assembly block is not recommended. BP is used to reference the
variables on the stack. If, for some reason, the BP register has to be
modified, it should be pushed on the stack first. Before the BP
register is popped back, referencing stack variables should avoided.
The following is an example:

/* sample program */
#include <stdio.h>

int nGlobal = 10 ;
void main (void)
{
int nStackVar = 1 ;

/* open the register window to observe the registers.
** open the debug window to observe the variables */
_asm {
        mov ax, nStackVar   ; nStackVar is referenced correctly
        push bp
        mov bp, sp          ; change bp
        mov ax, nStackVar   ; nStackVar is not right
        mov bx, nGlobal     ; nGlobal is OK
        pop bp
        mov ax, nStackVar   ; nStackVar is OK again
        }
}

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