Implement block-based heap

kernel/heap.asm

@@ -1,12 +1,14 @@
 heap_size:
-  dw 0x0100
+  ; 1 step here means 1 byte heap alloc table + 16 byte heap data
+  ; 0x0400 = 1kB table, 16kb data
+  dw 0x0400
 
 heap_init:
   push ax ; byte value (0)
   push cx ; loop counter
   push di ; target ptr
   push es
-  mov ax, ds
+  mov ax, cs
   mov es, ax
   xor ax, ax
   mov cx, [heap_size]
@@ -21,12 +23,62 @@ heap_init:
 ; in	ax	number of bytes to alloc
 ; out	ds:di
 malloc:
-  push dx ; length of block index
+  push cx ; number of blocks to allocate
-  or ax,0x0F
+  push bp ; base offset for block table
-  inc ax
+  push es
-  mov dx, 0x100
-  mov bp, heap
 
-  int 0x2E
+  ; transfer 12 MSB from ax to cx
-  pop dx
+  add ax, 0x000F
+  mov cl, 0x04
+  shr ax, cl
+  mov cx, ax
+
+  mov bp, heap
+  mov di, 0x0000
+  mov ax, ds
+  mov es, ax
+  xor ah, ah ; we use ax for reading from al once
+  jmp .firstblock
+
+.testlength:
+  push cx
+  push di
+  add di, bp ; scasb doesn't add bp on its own
+  repe scasb
+  pop di
+  pop cx
+  je .found
+  ; find next free item
+  mov al, 0x01
+.nextblock:
+  add di, ax
+.firstblock:
+  mov al, [bp+di]
+  cmp al, 0x00
+  jne .nextblock
+  cmp cx, 0x0001 ; fast case: 1 block allocation, skip test if length fits
+  jne .testlength
+.found:
+  mov [bp+di], cl
+  mov cl, 0x04
+  shl di, cl
+  add di, bp
+  add di, [heap_size]
+  pop es
+  pop bp
+  pop cx
+  ret
+
+; in	ds:di	begin of data block to free (di trashed)
+free:
+  push cx
+  push bp
+  mov bp, heap
+  sub di, [heap_size]
+  sub di, bp
+  mov cl, 0x04
+  shr di, cl
+  mov byte [bp+di], 0x00
+  pop bp
+  pop cx
   ret