Learning ASM, a basic question

ostapas · **Joined:** Thu Feb 18, 2010 11:10 pm **Posts:** 192

PB pushed me to learn some ASM and for obvious reasons I chose Fasm and use PB IDE and debugger for it. So far it goes not really smoothly

Could somenone tell me, how can I define a variable using Fasm data directives and debug it using PB's Debug Output? If I do like this:

Code:

! var db 255
Debug var

I just get an IMA.

luis · **Posted:** Wed Jul 31, 2013 7:43 pm

I'm not an ASM expert but I think it goes like this:

Debug var is trying to access the implicit declared "var" variable (since no enable explicit) and as explained in the docs that var will be internally know to PB (and fasm) as v_var, not var.
So is you remove your fasm line to prevent the crash, debug will print the value of the PB variable "var", mapped to "v_var" in fasm.

Moreover I think your "var db 255" is like poking a 255 binary number just in the middle of your PB program, in the code section. So the program will encounter a strange opcode when reaching that point and crash.

You can somewhat verify that by replacing 255 with 0x90, the opcode for NOP (no operation).

That is a valid opcode and it will be executed without crashing the program.

If I'm wrong please correct me.

ostapas · **Joined:** Thu Feb 18, 2010 11:10 pm **Posts:** 192

Many thanks for a reply!

Quote:

You can somewhat verify that by replacing 255 with 0x90, the opcode for NOP (no operation).

I think you was right, it was executing binary data.

Quote:

So is you remove your fasm line to prevent the crash, debug will print the value of the PB variable "var", mapped to "v_var" in fasm

Hm, didn't get your idea exactly, maybe you could elaborate on this a bit, if you got some spare minutes(I started to learn asm just today, so take it easy

)

luis · **Posted:** Wed Jul 31, 2013 8:30 pm

Debug var for PB means, "print the value of the var v_var" (pb append a v_ to variable names, l_ to labels, etc.).

So to read from you FASM data and print through debug, you could do something like this:

Code:

Define var ; this is the PB "var", it will translated to "v_var" to the ASM output to be sent to FASM

! mov eax, [var] ; this will copy your FASM "var" into a register
! mov [v_var], eax ; this will copy that value to the "v_var" the debug command can access

Debug var ; and here it is

End ; end to stop the program here

! var dd 1234567 ; your fasm data (double word to comply easily with register size)

But as I said, I'm not used to mix PB and ASM, so I'm sure you can get more appropriate answers from someone else.

ts-soft · **Posted:** Wed Jul 31, 2013 8:37 pm

You should change the first line of example from luis to:

Code:

Define var.l

or var is an integer and can't hold the dd with x64!

ostapas · **Joined:** Thu Feb 18, 2010 11:10 pm **Posts:** 192

luis, ts-soft, sincere thanks, exactly what I was looking for.

idle · **Posted:** Wed Jul 31, 2013 10:37 pm

as a little fasm processor debug experiment it's not very useful, x64 only

Code:

EnableASM 

var2 = 255 
var1 = 123 
mov rax, var2 
add rax,1
mov var2, rax

DisableASM

Global gDebug 

Macro debugASM(val)
   !  if val in <rax,rbx,rcx,rdx,rsp,rdi> 
   !     mov [v_gDebug],  val 
   !  else 
     !if defined v_#val
        ! push rax
        ! mov rax, [v_#val]
        !mov [v_gDebug], rax
        !pop rax 
    !else 
       !push rax
       !add rsp, 8
       !mov rax, [p.v_#val]
       !mov [v_gDebug], rax
       !sub rsp, 8 
       !pop rax 
     !end If    
  ! end If 
  Debug gDebug 
EndMacro 

Procedure foo()
   Protected b  
   b = 4 
  debugASM(b)
   
EndProcedure    

DebugASM(rax)
DebugAsm(var1)

foo()

ostapas · **Joined:** Thu Feb 18, 2010 11:10 pm **Posts:** 192

Quote:

as a little fasm processor debug experiment it's not very useful, x64 only

A bit advanced for me, but I suspect, this will come in handy later. Thanks!

Danilo · **Posted:** Thu Aug 01, 2013 8:53 am

luis wrote:

So to read from you FASM data and print through debug, you could do something like this:

Code:

Define var ; this is the PB "var", it will translated to "v_var" to the ASM output to be sent to FASM

! mov eax, [var] ; this will copy your FASM "var" into a register
! mov [v_var], eax ; this will copy that value to the "v_var" the debug command can access

Debug var ; and here it is

End ; end to stop the program here

! var dd 1234567 ; your fasm data (double word to comply easily with register size)

If your variable is within the code section, you are not allowed to write to it by default. Better put data/variables in a data section.

Code:

Define var.l ; this is the PB "var", it will translated to "v_var" to the ASM output to be sent to FASM

! mov eax, [var] ; this will copy your FASM "var" into a register
! mov [v_var], eax ; this will copy that value to the "v_var" the debug command can access

Debug var ; and here it is

! mov dword [var], 987654321 ; write to your var -> THIS WORKS ONLY IF YOUR VAR IS IN DATA SECTION

! mov eax, [var] ; this will copy your FASM "var" into a register
! mov [v_var], eax ; this will copy that value to the "v_var" the debug command can access
Debug var ; and here it is

End

DataSection
! var dd 1234567 ; your fasm data (double word to comply easily with register size)
EndDataSection

ostapas · **Joined:** Thu Feb 18, 2010 11:10 pm **Posts:** 192

Quote:

If your variable is within the code section, you are not allowed to write to it by default. Better put data/variables in a data section.

Thanks, understood.

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