Loading a moving offset into a reg

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moveax41h

Joined: 18 Feb 2018
Posts: 59

moveax41h 16 Mar 2018, 08:58

In the following code I am trying to check each char in a char array one at a time:

Code:

encode:
mov al, offset [bInput+ecx] ; Put the char* to first element into eax
cmp byte [eax], 32 ; check for space
je endme
cmp byte [eax], 97
jl not_lower_case
cmp byte [eax], 122
jg not_lower_case

There is code below this which does a comparison and then increments ecx before looping back up to the encode label.

However, something is wrong with my syntax and this first caused a segfault, now it won't even assemble. Is there a way to properly do this or do I need to use esi or edi? Thanks.

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-moveax41h

16 Mar 2018, 08:58

fasmnewbie

Joined: 01 Mar 2011
Posts: 555

fasmnewbie 16 Mar 2018, 09:46

[eax] is already a "char" by the second line. No longer a valid address.

What you need is a proper index addressing setup before going through the bytes.

But you could instead use "cmp al,32". Makes more sense.

16 Mar 2018, 09:46

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20519
Location: In your JS exploiting you and your system

revolution 16 Mar 2018, 11:07

Use the value in al directly. Also fasm supports ASCII characters natively, no need to convert to raw magic numbers. Plus use unsigned checks for unsigned values (jb and ja)

Code:

encode:
        mov     al, [bInput+ecx]        ; Put the char* to first element into al
        cmp     al, ' '
        je      endme
        cmp     al, 'a'
        jb      not_lower_case
        cmp     al, 'z'
        ja      not_lower_case

16 Mar 2018, 11:07

DimonSoft

Joined: 03 Mar 2010
Posts: 1228
Location: Belarus

DimonSoft 16 Mar 2018, 19:53

Also no need to use the offset keyword. It is just a useless junk invented by TASM/MASM (?) developers.

16 Mar 2018, 19:53

moveax41h

Joined: 18 Feb 2018
Posts: 59

moveax41h 17 Mar 2018, 22:31

Thank you, this was very helpful advice!

_________________
-moveax41h

17 Mar 2018, 22:31

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