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Should I preserve the general regs(eax,etc) in multitasking.
Posted: Tue Mar 09, 2021 6:33 pm
by clementttttttttt
Google doesn't seem to help this time.
Re: Should I preserve the general regs(eax,etc) in multitask
Posted: Tue Mar 09, 2021 7:03 pm
by kzinti
Assuming you are using ia32 + system V, you need to preserve ebp, edi, esi and ebx explicitely (and eip, esp implicitely).
Example:
Code: Select all
task_switch:
movl 4(%esp), %eax # eax = old context
movl 8(%esp), %edx # edx = new context
# Save old context
pushl %ebp
pushl %edi
pushl %esi
pushl %ebx
# Switch stacks
movl %esp, (%eax) # *old context = esp
movl %edx, %esp # esp = new context
# Restore new context
popl %ebx
popl %esi
popl %edi
popl %ebp
ret
Re: Should I preserve the general regs(eax,etc) in multitask
Posted: Tue Mar 09, 2021 7:40 pm
by bzt
clementttttttttt wrote:Google doesn't seem to help this time.
Depends if you have cooperative multitasking or pre-emptive multitasking.
With cooperative, your code actively yields. There you can save the registers with push/pop on the caller side, or you could just tell the compiler that registers might change (using
yield();asm(:::"rax, rbx, rcx, etc.");).
With pre-emptive multitasking, your code could be interrupted at any arbitrary point, so there you must save all registers on task switch (GPRs at a minimum, plus XMM if the task uses floating point arithmetic too).
Cheers,
bzt