SP decrement with Push

[arka.sharma]

Hi All,

If I use full segment definition and don't use .MODEL and USE16 or USE32 then how SP will be decremented in following cases

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push 0h

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var dd 0
push cs:var

Regards,
Arka

[TouEnMasm]

There is no direction flags for the push.
Are you sure of this ????????

[arka.sharma]

Direction flag usually comes into picture while copying block of data right ? Intel Reference Manual doesn't mention anything about direction flag for push I guess.

[jj2007]

There is no direction flags for the push.
Are you sure of this ????????

Me sure of this!!!!!!

[dedndave]

the direction flag is used with the "string" instructions
MOVS, CMPS, SCAS, STOS, LODS

as for pushing a dword in 16-bit segments, i suppose it's done with 2 pushes
the high word is pushed first, then the low word

if you push a constant, the size is determined by the segment USES

[FORTRANS]

Hi,

   Using MASM 5.0 and producing a listing, I got the following.

Code: [Select]

CODE    SEGMENT
   Pushing a constant or a double word variable generates an
error as they are not valid 8086 instructions.

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.386
CODE    SEGMENT
   Pushing a constant and a double word variable generates
valid 32-bit instructions in 32-bit code.

Regards,

Steve

[TouEnMasm]

perhaps a little more code like this one (old so old ...)

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                 ;Piece of code
                   
 DATA SEGMENT WORD PUBLIC 'DATA'
      PILE DW 0
            PDOSPASS STRUC
  DAX  DW ?
      PDOSPASS ENDS
 DATA ENDS
 
 DGROUP GROUP DATA
 CODE SEGMENT BYTE PUBLIC 'CODE'
 ASSUME CS:CODE,DS:DGROUP,ES:DGROUP,SS:DGROUP
            PUBLIC DOSPASS
   
 DOSPASS PROC FAR
                 MOV   PILE,SP
                 MOV   AX,SP
         PUSH  BP
                 SUB    AX,SP                 ;see here if it is negative (FF..h) or positive (2h) ?
         MOV   BP,SP                  ;debugger can be usefull!
         PUSH  AX
         PUSH  DS
         SAHF

         MOV   BX,[BP+6]

                 ;end
                 SAHF 
POP  DS
POP  AX
POP  BP
MOV  SP,PILE
RET 2
       DOSPASS ENDP
      CODE ENDS
   END


[BlueMR2]

as for pushing a dword in 16-bit segments, i suppose it's done with 2 pushes
the high word is pushed first, then the low word

That's right.  Intel is little endian.  I remember having to do DWORD pushes in DESQview.  On my 486 I could just use the 32-bit override, but on the 8088 I had to push high word, then low word (Quarterdeck also supplied macros for doing that to make life easier, like as used in https://github.com/BrianKnoblauch/DVClock/blob/master/CLOCK.ASM ).

[arka.sharma]

I performed the following test. I declare a dword variable in code segment as following

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var dd 0
Then I added following piece of code

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pusha
mov bp, sp
call _DumpRegister
push cs:var
mov bp, sp
add sp, 4
call _DumpRegister
popa
_DumpRegister is a routine which dumps all the registers in console, As it can be seen I have not used "dword ptr" for pushing "var", I have verified the content of bp and I found after the push it is decremented by 4. I have not declared any USE16 or USE32 in my module. I am thinking how the operand width is being calculated in this case. I am using MASM 6.1 and generated exe is MZ format.

[dedndave]

if you disassemble that program, you will find that two PUSH'es are used