rdos/kernel/malloc.asm

; All these functions assume DS=0

malloc_reserve_16:
  push bx
  call malloc_mark_allocated
  pop bx
  add bx, 0x10
  loop malloc_reserve_16
  ret

malloc_reset:
  xor ax, ax
  mov es, ax
  call malloc_get_table
  mov cx, 0x100 ; 256 words
  rep stosw

  ; reserve IVT and BDA
  xor bx, bx
  mov cx, 0x50
  call malloc_reserve_16

  ; reserve malloc table itself
  mov bx, 0x0600
  mov cx, 0x20
  call malloc_reserve_16

  ; reserve location for chainloaded boot sector
  mov bx, 0x7C00
  mov cx, 0x20
  call malloc_reserve_16
  ret

; Input:
;   0:BX - ptr to mem paragraph
; Output:
;   AL	- bitmask for byte in table
;   BX  - offset in table
malloc_calc_offsets:
  ; ch is bit offset
  ; cl is operand for bit shifts
  push cx

  ; strip last 4 bits (offset within paragraph)
  mov cl, 4
  shr bx, cl

  ; backup the next 3 bits of bx (wll be bit offset)
  mov ch, bl
  and ch, 0x07

  ; calculate byte offset by shifting off 3 bits
  mov cl, 3
  shr bx, cl

  ; setup bit mask in al, shifted by bit offset
  mov al, 0x01
  mov cl, ch
  shl al, cl

  pop cx
  ; fallthrough to setup DI

malloc_get_table:
  mov di, 0x0600
  ret

; IN: 0:BX ptr to paragraph to be marked as allocated
malloc_mark_allocated:
  call malloc_calc_offsets
  or byte [bx+di], al
  ret

; IN: 0:BX ptr to paragraph to be marked as free
malloc_mark_free:
  call malloc_calc_offsets
  not al
  and byte [bx+di], al
  ret