89 lines
2.1 KiB
NASM
89 lines
2.1 KiB
NASM
global alloc, free
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; metadata
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; +0: size
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; +8: next
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; +16 data
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section .bss
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free_list: resq 1 ; head of free list
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alloc_list: resq 1 ; head of allocation list
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section .text
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alloc:
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add rdi, 16 ; add space for metadata
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mov rax, [free_list] ; load head of free list
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xor r8, r8 ; last block
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.loop:
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test rax, rax ; end of list?
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jz .new_block ; yes? allocate new block
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cmp [rax], rdi ; does object fit in block and have space for metadata?
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jge .found_block ; yes? use this block
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mov r8, rax
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mov rax, [rax + 8] ; load next free block
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jb .loop ; no? go to next block
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.found_block:
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sub qword [rax], rdi ; reduce the available size of the block
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cmp qword [rax], 0 ; no space left in block?
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jg .done_remove_free_block ; no? do not remove block from free list
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mov rsi, [rax + 8] ; yes? remove block from free list
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test r8, r8 ; is current head of list?
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jz .remove_free_head ; yes? remove head
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mov [r8 + 8], rsi ; set prev.next to this.next
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jmp .done_remove_free_block
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.remove_free_head
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mov [free_list], rsi
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.done_remove_free_block:
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mov rsi, [rax] ; load size of block excluding the newly allocated object
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lea rax, [rax + rsi + 16] ; address of allocated block
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sub rdi, 16
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mov [rax], rdi ; save size
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mov rsi, [alloc_list] ; load head of allocated blocks
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mov [rax + 8], rsi ; move head to be next item after this block
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mov [alloc_list], rax ; set new head to this block
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lea rax, [rax + 16] ; skip metadata for return value
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ret
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.new_block:
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push rdi
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push r8
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mov rdi, 4096 ; page size
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call sys_mmap ; allocate a page
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mov qword [rax], 4080 ; set size of block to block size - metadata
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mov rsi, [free_list]
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mov qword [rax + 8], rsi ; move head to be the next item after this block
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mov [free_list], rax ; set new head to this block
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pop r8
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pop rdi
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jmp .found_block
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free:
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ret
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sys_mmap:
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mov rax, 9
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mov rsi, rdi
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mov rdi, 0
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mov rdx, 3
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mov r10, 34
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mov r8, -1
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mov r9, 0
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syscall
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cmp rax, -1
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je .error
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ret
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.error:
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mov rax, 60
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mov rdi, 1
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syscall
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sys_munmap:
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mov rax, 11
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syscall
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cmp rax, -1
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je .error
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ret
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.error:
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mov rax, 60
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mov rdi, 1
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syscall
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