Accessing 32bit assembly array, memory addressing

Question

Hi I'm new to assembly currently coding in 32bit assembly and I'm trying to access an array I initialized.

This is the array

lookup: dd 0, 3, 6, 9, 2, 5, 8, 1, 4, 7

This is what I'm trying to accomplish

add     bh, [lookup+al*4]

al is the index I want to add to bh

The code in c would be

b += a[i];

(b is bh, array is lookup, and al is i)

Any advice on how to accomplish this would be awesome, thanks!

al*4 isn't valid when specifying an address. Use eax*4 instead (and make sure that the unused bits of eax are zeroed). — Michael
– Michael, Commented Feb 18, 2015 at 20:23
Thank you for your help! However now when I try to mov eax, al I get an invalid number of opcode and operands error — tdwig
– tdwig, Commented Feb 18, 2015 at 20:37

Community · Accepted Answer · 2017-05-23 10:24:12Z

1

As Michael indicated in the comments, you can't use al in an effective address. Instead, we'll zero-extend al into eax, and use it instead:

movzx   eax, al                   ; zero-extend 8-bit al into 32-bit eax
add     bh, [lookup + eax*4]

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answered Feb 19, 2015 at 5:26

Jonathon Reinhart

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