Maths and Assembly

[FlySky]

Hey guys,

When having the following:

0131CA98  E8 238313FF    CALL 00454DC0   

Is it possible to use the opcodes to generate the address 00454DC0  manually?

Same goes for jmp:

0131CC22  ^E9 36FDFFFF      JMP 0131C95D

The compiler generates the appropriate distances for jumps and calls etc.. I am wondering if it is possible
to use the opcodes to generate the addresses manually.

[bomz]

use

Code Select Expand

..............
jmp MyPoint
..............
MyPoint:
...............
call MyProc
..............
MyProc proc
.............
than you assembly using Debug only manualy. without Debug you must use codes of operation instead of CALL JMP

[dedndave]

the answer is yes   :bg

these are known as "relative" addresses
that means that the operand that is encoded into the instruction is relative to the current instruction pointer
for jumps and calls, it is actually relative to the address of the following instruction
the operand is treated as a signed value and is stored in little-endian form
if the operand is positive, it is a forward branch
if the operand is negative, it is a backward branch

Code Select Expand

0131CA98  E8 238313FF    CALL 00454DC0
in this case, the address of the next instruction is 0131CA9D
the encoded operand is FF138323, a negative value
0131CA9D + FF138323 = 00454DC0

[FlySky]

Great reply dedndave ,

Exactly what I was looking for, thanks!!

[vanjast]

You'll find a lot of this in the Kernel.dll of windoze, and other dlls

[baltoro]

DAVE !!!
That is interesting as hell. How did you figure it out ???

[qWord]

this forum may need some irony-tags?

[baltoro]

:bg