At my university we were just introduced to IA32 x87 FPU. But we weren't informed how to clear the FPU-Stack of no longer demanded elements.
Imagine we're performing a simple calculation like (5.6 * 2.4) + (3.9 * 10.3).
.data
value1: .float 5.6
value2: .float 2.4
value3: .float 3.8
value4: .float 10.3
output: .string "The result is: %f\n"
.text
.global main
main:
fld value1 # Load / Push 5.6 into FPU
fmul value2 # Multiply FPU's top (5.6) with 2.4
fld value3 # Load / Push 3.8 into FPU
fmul value4 # Multiply the top element of the FPU's Stacks with 10.3
fadd %st(1) # Add the value under the top element to the top elements value
.output:
# Reserve memory for a float (64 Bit)
subl $8, %esp
# Pop the FPU's top element to the program's Stack
fstpl (%esp)
# Push the string to the stack
pushl $output
# Call printf function with the both parameters above
call printf
# Free the programs stack from the parameters for printf
addl $12, %esp
.exit:
movl $1, %eax
int $0x80
The problem is: After popping the FPU's top element which holds the calculation's result. How do I free the FPU's stack from the now remaining newly top element which holds the result of (5.6*2.4).
The only way I'm capable to imagine is freeing some more program stack and popping elements from the FPU's stack until all no longer demanded elements are removed.
Is there a way to directly manipulate the top pointer?
To accomplish that you don't have any garbadge on the stack you need to use the FADDP and FMULP and similar instructions.