CODEWITHSUNDEEP
assemblyboolean-logic
pokiman
pokiman

Reputation: 986

Tutorial on Boolean Logic as used in Assembly

Is there a tutorial somewhere which explains all the intricacies of boolean logic operations which are possible in assembly language. For example, I have no idea what the following assembly code block does:

mov     [ebp-8h], 31982853h
mov     eax, [ebp+Ch]
shl     eax, 8
mov     ecx, [ebp+Ch]
shr     ecx, 8
or      eax, ecx

I understand what the shl, shr and or instructions do but have no idea about their combined result. Can anybody direct me to a tutorial which thoroughly explains boolean logic operations in assembly?

Thanks.

Upvotes: 0

Views: 2996

Answers (2)

Alexey Frunze
Alexey Frunze

Reputation: 62106

If the variable at [ebp+Ch] holds a 16-bit value (i.e. only the 16 least significant bits can be non-zero), then this piece of code efectively swaps its bytes (bits 0 through 7 with bits 8 through 15). However, the result in eax after or eax, ecx has to be masked (and'ed) with 0FFFFh, unless the code that follows uses ax instead of eax.

Upvotes: 1

Scott Hunter
Scott Hunter

Reputation: 49893

Annotated code:

mov     eax, [ebp+Ch]   ; grab our 32-bit value
shl     eax, 8          ; shift it left 8 bits, so the top 8 bits are shifted out 
                        ; and the bottom 8 bits are filled w/ 0
mov     ecx, [ebp+Ch]   ; grab our 32-bit value again
shr     ecx, 8          ; shift this copy right 8 bits, so the bottom 8 bits are 
                        ; shifted out and the top 8 bits are filled w/ 0
or      eax, ecx        ; perform A = A or C for each set of same-aligned bits in
                        ; EAX (A) and ECX (C)

So if the original value was ABCD (each letter being an 8-bit byte), then before the or is performed, EAX would have BCD0 and ECX would have 0ABC.

Upvotes: 3

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