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Create context_switch.s

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new context_switch.s for x86 IA32.
must confirm nasm.
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Gregory Kenneth Bowne
2025-11-19 08:43:11 -08:00
committed by GitHub
parent 77400d8f5a
commit e9a78c835a
1 changed files with 25 additions and 0 deletions
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kernel/context_switch.s Normal file
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.global ctx_switch
; void ctx_switch(uint32_t **old_sp_ptr, uint32_t *new_sp);
; Arguments on stack (cdecl convention):
; [ESP + 4] -> old_sp_ptr (pointer to the 'stack_ptr' field of current task)
; [ESP + 8] -> new_sp (value of 'stack_ptr' of the next task)
ctx_switch:
; 1. Save the context of the CURRENT task
pushf ; Save EFLAGS (CPU status flags)
pusha ; Save all General Purpose Regs (EAX, ECX, EDX, EBX, ESP, EBP, ESI, EDI)
; 2. Save the current stack pointer (ESP) into the pointer passed as 1st arg
mov eax, [esp + 40] ; Get 1st argument (old_sp_ptr). Offset 40 = 36 (regs) + 4 (ret addr)
mov [eax], esp ; *old_sp_ptr = ESP
; 3. Load the stack pointer of the NEW task
mov esp, [esp + 44] ; Get 2nd argument (new_sp). Offset 44 = 40 + 4
; 4. Restore the context of the NEW task
popa ; Restore all General Purpose Regs
popf ; Restore EFLAGS
; 5. Jump to the new task (The 'ret' pops EIP from the new stack)
ret