161 lines
3.7 KiB
NASM
161 lines
3.7 KiB
NASM
global alloc, free
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section .bss
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free_list_head: resq 1 ; head of free list
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alloc_list_head: resq 1 ; head of allocation list
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section .text
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alloc:
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add rdi, 16 ; reserve 16 bytes for metadata
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mov rsi, [free_list_head]
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xor r8, r8
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.loop:
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test rsi, rsi ; allocate new block if end of list is reached
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jz .new_block
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mov rdx, [rsi]
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cmp rdx, rdi ; is there enough space for allocation?
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ja .use_block ; yes? allocate
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add rdx, 16
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cmp rdx, rdi ; no? is there enough space if we include metadata
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je .use_block ; if we include metadata, the sizes has to match exactly, or partial metadata will persist
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mov r8, rsi ; r8 contains the node from the last iteration
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mov rsi, [rsi + 8] ; next node
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jmp .loop
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.new_block:
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push rdi
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push r8
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add rdi, 16
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mov rsi, 4096
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call max
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push rax
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mov rdi, rax
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call sys_mmap
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pop rsi
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sub rsi, 16
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mov qword [rax], rsi ; update metadata to page size - metadata
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push rax
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mov rdi, rax
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call insert_into_free
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pop rsi
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pop r8
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pop rdi
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.use_block:
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cmp [rsi], rdi ; check if the block will be empty after allocation
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ja .unlink_done ; if not, do not unlink
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test r8, r8 ; r8 is null if node is also head
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jz .unlink_head
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mov rdx, [rsi + 8] ; load next node
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mov [r8 + 8], rdx ; link next node to last node's next
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jmp .unlink_done
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.unlink_head:
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mov rdx, [free_list_head] ; load head
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mov rdx, [rdx + 8] ; load head.next
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mov [free_list_head], rdx ; mov head.next into head
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.unlink_done:
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sub [rsi], rdi ; reduce available space of block by the allocated space
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mov rdx, [rsi] ; load the available space excluding the newly allocated space
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lea rax, [rsi + rdx + 16] ; load the address of the newly allocated space
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sub rdi, 16
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mov [rax], rdi ; update metadata to allocation size - metadata
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mov rdx, [alloc_list_head]
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mov [rax + 8], rdx ; move head to nex item in this alloc
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mov [alloc_list_head], rax ; update head to point to this node
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lea rax, [rax + 16] ; skip past metadata for return value
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ret
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free:
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lea rdi, [rdi - 16] ; adjust for metadata
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mov rsi, [alloc_list_head]
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xor r8, r8
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.loop:
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test rsi, rsi
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jz .not_found
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cmp rdi, rsi
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je .found
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mov r8, rsi
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mov rsi, [rsi + 8] ; next node
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jmp .loop
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.not_found:
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mov rax, 60
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mov rdi, 1
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syscall
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.found:
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test r8, r8 ; r8 is null if node is also head
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jz .unlink_head
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mov rdx, [rsi + 8] ; load next node
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mov [r8 + 8], rdx ; link next node to last node's next
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jmp .unlink_done
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.unlink_head:
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mov rdx, [alloc_list_head] ; load head
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mov rdx, [rdx + 8] ; load head.next
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mov [alloc_list_head], rdx ; mov head.next into head
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.unlink_done:
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mov rdi, rsi
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call insert_into_free
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ret
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insert_into_free:
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mov rsi, [free_list_head] ; load head
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test rsi, rsi ; is list empty
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jz .insert_head ; if empty, insert at head
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cmp rdi, rsi ; is input smaller then head
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jl .insert_head ; if smaller, insert at head
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xor r8, r8 ; r9 will track the previous node
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.loop:
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test rsi, rsi
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jz .insert_end ; if at end of list, insert at end
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cmp rdi, [rsi + 8] ; compare input to next node
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jg .next ; if larger, skip to next node
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mov [rsi + 8], rdi ; if smaller, insert at this position
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mov [rdi + 8], rdx
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ret
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.next:
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mov r8, rsi ; update r8 to current node
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mov rsi, [rsi + 8] ; update rsi to next node
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jmp .loop
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.insert_head:
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mov rdx, [free_list_head]
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mov [rdi + 8], rdx
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mov [free_list_head], rdi
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ret
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.insert_end:
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mov [r8 + 8], rdi ; update last node's next to point at rdi
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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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max:
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cmp rdi, rsi
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jae .left
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mov rax, rsi
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ret
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.left:
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mov rax, rdi
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ret
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