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Preserving EBX register
Posted: Sun Apr 26, 2015 5:15 pm
by Inf0Byt3
I've got this procedure where I need an extra register besides eax, ecx and edx. ebx seems to be good for the task, but I can't seem to be able to preserve it. For example consider the following code:
Code: Select all
Procedure myproc(bbb.l)
CompilerIf #PB_Compiler_Processor = #PB_Processor_x64
!push rbx
CompilerElse
!push ebx
CompilerEndIf
;bbb + 1
!mov ebx, dword [p.v_bbb]
!inc ebx
!mov dword [p.v_bbb], ebx
CompilerIf #PB_Compiler_Processor = #PB_Processor_x64
!pop rbx
CompilerElse
!pop ebx
CompilerEndIf
ProcedureReturn bbb
EndProcedure
n = myproc(5) ;should return 6
Debug n
If the push and pop parts are commented, the result comes out ok. What is the safe way to use ebx?
Re: Preserving EBX register
Posted: Sun Apr 26, 2015 5:31 pm
by wilbert
When you alter the stack with pushing and popping registers, that messes up references to local variables like [p.v_bbb]
What I'm currently using is an integer variable to store the register. This works fine.
The same way you can preserve
rsi/esi and
rdi/edi if you need even more registers.
Using
rbp/ebp as an extra register causes problems with the debugger.
Code: Select all
Procedure myproc(bbb.l)
Protected.i reg_bx
CompilerIf #PB_Compiler_Processor = #PB_Processor_x64
!mov [p.v_reg_bx], rbx
CompilerElse
!mov [p.v_reg_bx], ebx
CompilerEndIf
;bbb + 1
!mov ebx, dword [p.v_bbb]
!inc ebx
!mov dword [p.v_bbb], ebx
CompilerIf #PB_Compiler_Processor = #PB_Processor_x64
!mov rbx, [p.v_reg_bx]
CompilerElse
!mov ebx, [p.v_reg_bx]
CompilerEndIf
ProcedureReturn bbb
EndProcedure
n = myproc(5) ;should return 6
Debug n
Re: Preserving EBX register
Posted: Sun Apr 26, 2015 5:35 pm
by Tenaja
Pb uses the stack for local variables. Since it doesn't use a frame pointer, any stack changes you make will kill the reference point.
This was a design decision the developer made early on, to allow more registers for optimizing, but at the cost of capabilities like this (and many other standard practices).
Re: Preserving EBX register
Posted: Sun Apr 26, 2015 6:33 pm
by Inf0Byt3
Thank you both for the explanation, it's perfectly clear now. I'll save the register in a variable as advised. Still got many things to learn
.