CODEWITHSUNDEEP
assemblyx86memory-segmentation
Total Anime Immersion
Total Anime Immersion

Reputation: 471

Change CS:IP in Assembly Language

How can you forcibly change CS and IP both in assembly language ?

ORG directive can be used to change the number of the IP, but how do you change the CS?

Basically I wan to implement multi-threading using assembly.

Many forums, including a question in stack overflow has said its impossible, but then how does C have multi-threading options even when it is made from assembly code ?

Upvotes: 2

Views: 6624

Answers (4)

Akib Azmain Turja
Akib Azmain Turja

Reputation: 1180

To change both cs and ip registers, use the following in AT&T syntax:

ljmp $segment, $offset

Or the following in Intel syntax:

jmp segment:offset

Upvotes: 1

user922475
user922475

Reputation:

Only far control transfer instructions (jmp, call, ret) can change the CS or EIP registers. I think the CS register can only be changed in real mode.

RET basically takes the value at ESP (stack pointer) and pushes that onto IP/EIP. Then the ESP is incremented by 8 plus the immediate offset (if exists).

Upvotes: 1

nrz
nrz

Reputation: 10570

To change cs:ip just make a long jump with jmp (eg. jmp segment:offset) or a long call (eg. call segment:offset) depending on your needs. There are several different addressing modes available for jmp link and call link. Implementing multithreading is a totally different matter from simply changing cs:ip.

Upvotes: 7

Pyjong
Pyjong

Reputation: 3197

I think you could use just 

jmp segment:offset

call segment:offset

Upvotes: 3

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