97 lines
2.7 KiB
NASM
97 lines
2.7 KiB
NASM
extern current
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global switchProcess
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struc PCB
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.PID resd 1 ; Offset: 0
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.STATE resd 1 ; Offset: 4
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.PROGRAM_COUNTER resd 1 ; Offset: 8
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.STACK_BASE resd 1 ; Offset: 12
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.STACK_POINTER resd 1 ; Offset: 16
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.FLAGS resd 1 ; Offset: 20
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.NEXT resd 1 ; Offset: 24
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endstruc
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section .text
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; void switchProcess(Process* next)
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switchProcess:
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; __________Stack___________
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; |------------------------|
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; | Return Address (EIP) | +0
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; |------------------------|
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; | Process* next | +4
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; |------------------------|
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; Push volatile registers (per the 32-bit System V ABI)
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push ebx
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push esi
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push edi
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push ebp
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; __________Stack___________
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; |------------------------|
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; | EBP | +0
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; |------------------------|
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; | EDI | +4
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; |------------------------|
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; | ESI | +8
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; |------------------------|
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; | EBX | +12
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; |-------- ---------------|
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; | Return Address (EIP) | +16
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; |------------------------|
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; | Process* next | +20
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; |------------------------|
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; Save current process' state
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mov edi, [current]
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; EDI now contains the pointer to [Process* current]
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; |-----|
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; | EDI | <== [Process* current]
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; |-----|
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; Set [current->PCB.STACK_POINTER] to the value in ESP
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; Process* current->PCB.STACK_POINTER <== ESP
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mov [edi + PCB.STACK_POINTER], esp
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; Load next process' state
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; ESP has been decremented by 16, so instead of +4,
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; we need to do +20. 4 pushes, 1 return address, 4
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; bytes each.
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mov esi, [esp + 20]
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; ESI now contains the pointer to [Process* next]
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; __________Stack___________
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; |------------------------|
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; | Return Address (EIP) | +16
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; |-----| |------------------------|
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; | ESI | <== | Process* next | +20
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; |-----| |------------------------|
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; Set [EXTERN Process* current] to the value in ESI
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; Process* current <== ESI (Process* next)
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mov [current], esi
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mov esp, [esi + PCB.STACK_POINTER]
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; |-----| |-----|
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; | ESP | <== | ESI |
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; |-----| |-----|
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; Restore non-volatile registers (per the 32-bit System V ABI)
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pop ebp
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pop edi
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pop esi
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pop ebx
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; The stack is now restored to its original state.
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; __________Stack___________
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; |------------------------|
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; | Return Address (EIP) | +0
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; |------------------------|
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; | Process* next | +4
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; |------------------------|
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ret
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