66 lines
1.7 KiB
NASM
66 lines
1.7 KiB
NASM
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%define mcb_first 0x5F
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; This alloc uses an MCB-like list always starting at a fixed offset.
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; Boot components may use it to allocate space during early boot.
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; The list ends with the most recently allocated arena at its end,
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; contrary to how the MCBs work on DOS.
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; Memory Layout:
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; 0000 Interrupt table
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; 0400 BIOS data area
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; 05F0 16 bytes Arena header 1
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; 0600 BX paragraphs Arena 1
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; .... 16 bytes Arena header 2
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; .... BX paragraphs Arena 2
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; ...
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; .... 16 bytes Arena header n
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; .... BX paragraphs Arena n
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; Unmanaged memory
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; Take note that this mechanism works differently than under DOS -
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; but this one is optimized for fast and small alloc.
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; Typical usecases:
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; - Diskette Parameter Table (See interrupt 1Eh)
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; - Partition table
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; - Boot sector relocations
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; - Boot sector data (FAT table, rootdir)
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; Allocate paragraphs in unmanaged memory
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; IN BX requested memory in paragraphs
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; OUT AX output segment
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early_alloc:
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push ds
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push bx
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push si
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; si is used as zero register
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; this allows the opcodes to use a byte as disp instead of word
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xor si, si
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mov ax, mcb_first
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.next:
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mov ds, ax
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inc ax
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; DS is seg of arena header
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; AX is seg of arena
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cmp BYTE [si], 0x4D
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jne .here
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add ax, WORD [si+3]
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jmp .next
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.here:
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; mark as item
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mov BYTE [si], 0x4D
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; write segment length in paragraphs
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mov WORD [si+3], bx
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; if allocation is higher than CS, then we are in managed memory
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; set ourself as owner then, otherwise use dummy value 8
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mov bx, cs
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cmp ax, bx ; CF = CS in unmanaged memory
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jnc .setup_owner
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mov bx, 8
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.setup_owner:
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mov WORD [si+1], bx
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pop si
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pop bx
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pop ds
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ret
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