🚀 go-pugleaf

>>>>> On Wed, 13 Jan 1993 20:56:44 GMT, riouxm@JSP.UMontreal.CA (Rioux Martin) said:
Rioux> 1. OK, Here's my question, Is it possible to use (EAX EBX ...) with the FPU
Rioux> commands ex.( FMUL EAX ) IF not how can you speed up the process..?

CPU and FPU communicate through I/O ports (286/287 or better) F0..FF,
but I don't know if it is possible to use these ports by instructions
to send EAX directly to FPU. Seems opcode should be send to port F8,
operand to FC (or 800000F8,800000FC on 386). But I couldn't get it
working. To get maximum speed without register to FPU transfer: store
EAX in memory at doubleword-aligned address using register (e.g. BX)
as address: mov [BX],EAX followed by FILD [BX]; note conversion from
integer to FPU-s internal form takes about 10 times more time than
data transfer from CPU to memory and from memory to FPU. 

Why you need transfer data directly from CPU to FPU ?
They use different data formats.
FPU loads 32-bit REAL in 20 clock cycles, 64-bit REAL in 25,
while loading 32-bit INTEGER takes 45 and 16-bit - 61!
Note CPU can access memory in 2-3 clock cycles. :-)

Rioux> 2. When I use FMUL TEMP,ST(3) I get an ( extra character on line ) I think 
Rioux> I used that line before ...!!! but now it doesn't work anymore... HELP!

Opcode specifying any of ST() other than ST(0) cannot specify memory
address or CPU register because ST index uses same opcode bits as
memory R/M field specifying memory addressing mode.
Can:	FMUL TEMP - multiply ST(0) by REAL TEMP
	FIMUL TEMP - multiply ST(0) by INTEGER TEMP
	FMUL ST(i) - multiply ST(0) by ST(i)
	FMUL ST(i),ST(0) - multiply ST(i) by ST(0)
Result comes to ST(0) except the last when it comes to ST(i).

Jerzy Tarasiuk <jt@fuw.edu.pl>