Add lib function for MCB allocation from bootsectors

lib/earlymcb.asm

@@ -0,0 +1,65 @@
+%define mcb_first 0x5F
+
+; This alloc uses an MCB-like list always starting at a fixed offset.
+; Boot components may use it to allocate space during early boot.
+; The list ends with the most recently allocated arena at its end,
+; contrary to how the MCBs work on DOS.
+
+; Memory Layout:
+;   0000                 Interrupt table
+;   0400                 BIOS data area
+;   05F0  16 bytes       Arena header 1
+;   0600  BX paragraphs  Arena 1
+;   ....  16 bytes       Arena header 2
+;   ....  BX paragraphs  Arena 2
+;                        ...
+;   ....  16 bytes       Arena header n
+;   ....  BX paragraphs  Arena n
+;                        Unmanaged memory
+
+; Take note that this mechanism works differently than under DOS -
+; but this one is optimized for fast and small alloc.
+
+; Typical usecases:
+; - Diskette Parameter Table (See interrupt 1Eh)
+; - Partition table
+; - Boot sector relocations
+; - Boot sector data (FAT table, rootdir)
+
+; Allocate paragraphs in unmanaged memory
+; IN	BX	requested memory in paragraphs
+; OUT	AX	output segment
+early_alloc:
+  push ds
+  push bx
+  push si
+  ; si is used as zero register
+  ; this allows the opcodes to use a byte as disp instead of word
+  xor si, si
+  mov ax, mcb_first
+.next:
+  mov ds, ax
+  inc ax
+  ; DS is seg of arena header
+  ; AX is seg of arena
+  cmp BYTE [si], 0x4D
+  jne .here
+  add ax, WORD [si+3]
+  jmp .next
+.here:
+  ; mark as item
+  mov BYTE [si], 0x4D
+  ; write segment length in paragraphs
+  mov WORD [si+3], bx
+  ; if allocation is higher than CS, then we are in managed memory
+  ; set ourself as owner then, otherwise use dummy value 8
+  mov bx, cs
+  cmp ax, bx ; CF = CS in unmanaged memory
+  jnc .setup_owner
+  mov bx, 8
+.setup_owner:
+  mov WORD [si+1], bx
+  pop si
+  pop bx
+  pop ds
+  ret