boolean odd / even

[Turtle16]

I am new to MASM and need help using boolean instructions to determine if a number input is even or odd. xor? test? etc...Any help would be appreciated.

[Petroizki]

Just test for the first bit.

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test eax, 1 ; the same as 'and', but doesn't modify eax
jnz odd
; even

The numbers go like this in bits:
0001 ; 1
0010 ; 2
0011 ; 3
0100 ; 4
0101 ; 5
etc...

[AeroASM]

Alternatively you could zero the first bit and see if is the same:

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;save eax on stack
push eax
;zero the first bit
shr eax,1
shl eax,1
;test if same
pop ecx
cmp eax,ecx
je even
odd:
;
even:
;


[roticv]

Alternatively you could zero the first bit and see if is the same:

Code Select Expand


;save eax on stack
push eax
;zero the first bit
shr eax,1
shl eax,1
;test if same
pop ecx
cmp eax,ecx
je even
odd:
;
even:
;


Why not just check the carry flag?

[The_Grey_Beast]

The best ways I know are:

1) like Petroizki said:

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test ax, 1
jnz odd
even:
; even code

odd:
; odd code

2)

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shr ax, 1
jc odd
even:
; even code

odd:
; odd code

The second one uses a shift, which modifies eax (divides it by 2). Also, it may be slower on processors such as 80x86, 286 or 386, since a shift is actually slower than an and operation (or test), but may be shorter (I think, correct me if I'm wrong).

[AeroASM]

It is shorter:

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test eax,1
assembles to

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A9,01,00,00,00

whereas

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shr eax,1
assembles to

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D1,E8

However, shr is twice as slow (on my computer (Pentium M 1.5GHz), 100 x test eax,1 takes 41 clock cycles but 100 x shr eax,1 takes 90 clock cycles)

[Turtle16]

Thanks to everyone who replied. You have been a great help.