jmp instruction encoding?

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silkodyssey

Joined: 02 Oct 2003
Posts: 198
Location: St.Vincent & the Grenadines

silkodyssey 30 Nov 2003, 20:42

Looking at a program with a debugger it seems that jmp is a two byte instruction. The first byte being EB and another byte. What is the other byte for? I thought it was the [destaddress] -/+ [currentaddress] but when I do the calculations everything doesn't always add up. Can anyone help me out with this?

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silkodyssey

30 Nov 2003, 20:42

comrade

Joined: 16 Jun 2003
Posts: 1150
Location: Russian Federation

comrade 30 Nov 2003, 21:40

EB (dest-src-2)

Code:

abc: jmp $ = EB (abc-abc-2) = EBFE

also there is E9 (long jmp), same as EB but for much farther ranges. Same idea.

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comrade (comrade64@live.com; http://comrade.ownz.com/)

30 Nov 2003, 21:40

Tomasz Grysztar

Joined: 16 Jun 2003
Posts: 8390
Location: Kraków, Poland

Tomasz Grysztar 30 Nov 2003, 21:45

The address put in the code of jump instruction is just relative to the next instruction - the one following the whole jump, which would be executed if jump was not taken.

30 Nov 2003, 21:45

silkodyssey

Joined: 02 Oct 2003
Posts: 198
Location: St.Vincent & the Grenadines

silkodyssey 30 Nov 2003, 23:32

Thanks for the info Comrade, Privalov. I guess I had the right idea in the first place. Smile

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silkodyssey

30 Nov 2003, 23:32

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