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Assembly language help

 
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Pencil3990
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PostPosted: Sun Jan 05, 2014 4:16 pm    Post subject: Assembly language help Reply with quote

What does the jmp command in Assembly language do? Im a bit new into assembly and had no luck researching what the jmp command does. Iv'e heard it Jumps to an address but i never got what "Jump" meant can some1 help Cool
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daspamer
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PostPosted: Sun Jan 05, 2014 5:46 pm    Post subject: Reply with quote

jumps to specific memory region.
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Chris12
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PostPosted: Mon Jan 06, 2014 3:20 am    Post subject: Reply with quote

Well it means that the processor will continue executing the code of the jump address.
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zm0d
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PostPosted: Mon Jan 06, 2014 10:45 am    Post subject: Reply with quote

it jumps the give number of bytes from the JMP-opcode address.
that means startaddress+amount of bytes to jmp = endaddress (called a relative jmp) Smile

Please, don't think you JMP by hard addresses Razz Smile
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justa_dude
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PostPosted: Mon Jan 06, 2014 4:33 pm    Post subject: Reply with quote

zm0d wrote:
Please, don't think you JMP by hard addresses :P :)

You can, though. There are jmp instructions that take as an operand an address instead of an offset. Most BIOS calls use interrupts, but for example there are a few (like the shutdown procedure, as I recall) that you simply jump to.
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zm0d
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PostPosted: Thu Jan 09, 2014 8:06 am    Post subject: Reply with quote

Oh yeah you're right Smile
using 0xEA (far jmp) instead of 0xE9 (relative jmp) is for an absolute jump Smile it takes an address as argument Razz that's nice and save the calculation to your jump Smile

http://ref.x86asm.net/geek.html#xEA
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pellik
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PostPosted: Wed Jan 15, 2014 11:17 am    Post subject: Reply with quote

Also it's worth noting that whatever debugger your using will figure out whether you need near or far jump automatically if you just put in jmp addr. So you need to be aware that the size of your far jump is likely a lot greater then the near jmp you might want to overwrite.
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