Introduce new memory allocator, now with the ability to free memory!!
This is a super simple linked list memory allocator, written with lots of help from my friend hyenasky.
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Ry
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70fd0823dd
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9d510e51b7
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@ -13,3 +13,4 @@ end_current_task: jmp [0x00000838]
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; memory jump table
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allocate_memory: jmp [0x00000840]
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free_memory: jmp [0x00000844]
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@ -1,35 +1,191 @@
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; memory allocator routines
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; this is a very basic memory allocator, it doesn't even allow freeing
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; TODO: make this better
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; block header fields:
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; data.32 size - size of block, NOT INCLUDING THE HEADER
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; data.32 prev - pointer to previous free block, or zero
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; data.32 next - pointer to next free block, or zero
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const HEADER_SIZE: 12
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const MEMORY_TOP: 0x02000000
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initialize_allocator:
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push r0
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push r1
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mov [free_list_head], kernel_bottom
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; set the free block size to MEMORY_TOP - [free_list_head]
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mov r0, [free_list_head]
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mov r1, MEMORY_TOP
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sub r1, r0
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mov [r0], r1
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; mark this as the only free block
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add r0, 4
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mov [r0], 0
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add r0, 4
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mov [r0], 0
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pop r1
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pop r0
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ret
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; allocate a block of memory
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; inputs:
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; r0: size in bytes
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; outputs:
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; r0: pointer to allocated block (or zero if no blocks free)
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; r0: pointer to allocated block (or zero if no blocks free of requested size)
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allocate_memory:
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push r1
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push r10
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push r11
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mov [block], [free_list_head]
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; r10: real_size = requested size + header size
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mov r10, r0
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add r10, HEADER_SIZE
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allocate_memory_while_block:
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mov r0, [block]
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call block_get_size
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cmp r0, r10
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ifgteq jmp allocate_memory_good_block
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; block = block->next
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mov r0, [block]
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call block_get_next
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mov [block], r0
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cmp [block], 0
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ifnz jmp allocate_memory_while_block
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; if we reach this point, no good blocks were found
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mov r0, 0
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pop r11
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pop r10
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pop r1
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ret
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allocate_memory_good_block:
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mov r11, r10
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add r11, HEADER_SIZE
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add r11, 16
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mov r0, [block]
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call block_get_size
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cmp r0, r11
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ifgt jmp allocate_memory_good_block_carve
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; next = block->next
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; prev = block->prev
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mov r0, [block]
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call block_get_next
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mov [next], r0
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mov r0, [block]
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call block_get_prev
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mov [prev], r0
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cmp [next], 0
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ifnz call allocate_memory_good_block_set_nextprev
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cmp [prev], 0
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ifnz jmp allocate_memory_good_block_set_prevnext
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mov [free_list_head], [next]
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allocate_memory_good_block_ret:
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mov r0, [block]
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add r0, HEADER_SIZE
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pop r11
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pop r10
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pop r1
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ret
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allocate_memory_good_block_carve:
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; block->size -= real_size
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mov r0, [block]
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call block_get_size
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sub r0, r10
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mov r1, r0
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mov r0, [block]
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call block_set_size
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; block += block->size
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mov r0, [block]
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call block_get_size
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add [block], r0
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; block->size = real_size
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mov r0, [block]
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mov r1, r10
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call block_set_size
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jmp allocate_memory_good_block_ret
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allocate_memory_good_block_set_nextprev:
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; next->prev = prev
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mov r0, [next]
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mov r1, [prev]
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call block_set_prev
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ret
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allocate_memory_good_block_set_prevnext:
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; prev->next = next
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mov r0, [prev]
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mov r1, [next]
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call block_set_next
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jmp allocate_memory_good_block_ret
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block: data.32 0
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next: data.32 0
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prev: data.32 0
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block_get_size:
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mov r0, [r0]
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ret
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block_set_size:
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mov [r0], r1
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ret
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block_get_prev:
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add r0, 4
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mov r0, [r0]
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ret
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block_set_prev:
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add r0, 4
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mov [r0], r1
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ret
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block_get_next:
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add r0, 8
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mov r0, [r0]
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ret
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block_set_next:
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add r0, 8
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mov [r0], r1
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ret
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; free a block of memory
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; inputs:
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; r0: pointer to allocated block
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; outputs:
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; none
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free_memory:
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push r0
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push r1
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push r2
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mov r1, MEMORY_TOP
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mov r2, [free_base]
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sub r1, r2
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cmp r1, r0
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iflteq jmp allocate_memory_full
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mov r1, [free_base]
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add [free_base], r0
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mov r0, r1
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pop r2
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pop r1
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ret
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allocate_memory_full:
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mov r0, 0
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; point to the header
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sub r0, HEADER_SIZE
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mov r2, r0
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; add it to the free list
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mov r1, 0
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call block_set_prev
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mov r0, r2
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mov r1, [free_list_head]
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call block_set_next
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cmp [free_list_head], 0
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ifnz mov r1, r0
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ifnz mov r0, [free_list_head]
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ifnz call block_set_prev
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mov [free_list_head], r2
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pop r2
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pop r1
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pop r0
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ret
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free_base: data.32 kernel_bottom
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free_list_head: data.32 kernel_bottom
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@ -31,6 +31,7 @@ jump_table:
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; memory jump table
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org.pad 0x00000840
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data.32 allocate_memory
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data.32 free_memory
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; initialization code
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entry:
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@ -38,7 +39,10 @@ entry:
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mov r0, BACKGROUND_COLOR
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call fill_background
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draw_startup_text:
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; initialize the memory allocator
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call initialize_allocator
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; draw the startup text
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mov r0, startup_str
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mov r1, 16
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mov r2, 464
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