Explain how the FFREE instruction works

Question

http://en.wikipedia.org/wiki/X86_instruction_listings

how does the ffree instruction excatly works ? FFREE Free register when i have in FPU STack:

  st2 5 , st1 2, st0 7

then after these instruction(ffree) i have..

   st 1 5, st0 2

is that right?

score 0 · Accepted Answer · answered Jan 12 '14 at 22:26

0

No, it clears the register without modifying the stack pointer. Whatever register you gave as an argument will be set as empty, but the others will be unchanged.

http://www.website.masmforum.com/tutorials/fptute/fpuchap3.htm#ffree

answered Jan 12 '14 at 22:26

Jens Björnhager

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hm, so in my example, if i use Ffree st1 , then after that i have: st2 5 , st1 0??, st0 7 – Kamil Witkowski Jan 12 '14 at 23:02
ALmost, it will not contain 0, it will contain garbage. – Jens Björnhager Jan 12 '14 at 23:06
so will i be able to use it again ? Or first i have to make sure that i turn st1 to '0' ? with some kind of operation ? For example, i would like to have st1 '1' after ffree st1 operation – Kamil Witkowski Jan 12 '14 at 23:15
You'll need to load something into it to use it again. If you want st1 to contain a value, why do you want to ffree it? – Jens Björnhager Jan 12 '14 at 23:26
no, no i only ask of my curiosity. then for example i can load into it variable jedynka: dd 1.0 ? – Kamil Witkowski Jan 12 '14 at 23:52
You can't load anything into it unless the spot is st0 – Jens Björnhager Jan 13 '14 at 00:06
so for example i first use fxch st1 – Kamil Witkowski Jan 13 '14 at 21:02

Explain how the FFREE instruction works

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