Help with FISTP and FILD interpretation

Question

I need some help with understanding what is being actualy done in those two assembly lines:

fistp qword ptr [esp+10h] ss:0023:0013be80=0000000000001cf8

fild qword ptr [esp+10h] ss:0023:0013be80=0000000000002c36

What is the underlying operetion that changes 1cf8 to 2c36

Thanks in adv

Intel's floating point instructions are based on a stack. fist stores a 16-32-64-bit floating point number (IEEE) to the operand address, whereas fild loads from an address into the stack. The p suffix, is the variation of fist that pops the first value, ST(0), from said stack. The values you're seeing are not the underlying operand changing the address, but rather that before fistp is run, the value is 0x1cf8. After executing fistp, however, ST(0) is popped and written to [esp+0x10] as 0x2c36. When execution gets to fild, the previous value of ST(0) was already written. — Ali Rizvi-Santiago, Commented Aug 23 at 18:34
...that is assuming that there's no other instructions tampering with [esp+0x10] in between the two instructions you listed. (Related question: reverseengineering.stackexchange.com/questions/20559/…, link for answer is archived at web.archive.org/web/20071006182549/http://www.ray.masmcode.com/…) — Ali Rizvi-Santiago, Commented Aug 23 at 18:40
First of all, sorry for late reply. Yes, you are right, it all seems clear for me now. Thank you for explanation. — Uddie, Commented Aug 29 at 19:59

blabb · Accepted Answer · 2024-08-23 19:36:24Z

volume 2 is instruction set reference

search fistp or any other instruction and read for better understanding

basically FISTP takes what is in st[0] converts it to a signed integer and stores it in destination and pops the st[0] (empties st[0])

suppose st[0] contains a valid float like 486.87 (using 32 bit ollydbg here)

and your instruction pointer contains fistp qword ptr [esp+10]

on executing this instruction esp+10 will hold 487 (486.87 rounded to 487 and positive signed) prior to execution

post execution

an the stack has been popped

hope you will be able to follow this for FILD

So sorry for late reply. I've got the procedure. I can manage on my own now. Thanks a lot! — Uddie, Commented Aug 29 at 20:02

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