CODEWITHSUNDEEP
assemblyx86flags
monkey doodle
monkey doodle

Reputation: 700

Will this set of instructions jump?

Will this set of instructions jump? My main concern is the CMP expression. Does CMP handle only unsigned numbers? if so it would jump, since the Zero flag would have been set. Otherwise it would not jump since the numbers are specifically different. 

   mov  ax, -127d
   cmp  ax, 128d
   je   Target

Upvotes: 1

Views: 84

Answers (2)

gsg
gsg

Reputation: 9377

cmp actually handles both signed and unsigned arithmetic simultaneously. After cmp executes the overflow, carry, sign and zero flags will all be suitably set: any interpretation of these flags is up to the subsequent conditional instruction(s).

For example jz/jnz are insensitive to sign, ja/jb implies unsigned comparison, and jl/jg implies signed comparison. See an x86 manual for the (extensive) details of condition codes.

Upvotes: 1

user555045
user555045

Reputation: 64904

Equality is sign-oblivious.

-127 = FF81 = 65409
 128 = 0080 =   128

Numbers that are unequal are unequal both in signed and unsigned interpretations. This follows directly from the fact that subtraction is the same procedure for signed and unsigned - since it's the same procedure, it can't, for the same inputs, result in 0 for one interpretation and not-0 for the other.

Upvotes: 2

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