Add malloc

kernel/main.asm

@@ -98,4 +98,8 @@ start:
 %include "intr.asm"
 %include "debug.asm"
 
+%include "malloc.asm"
+
+times 512 nop
+
 align 512

kernel/malloc.asm

@@ -0,0 +1,54 @@
+; All these functions assume DS=0
+
+malloc_reset:
+  xor ax, ax
+  mov es, ax
+  call malloc_get_table
+  mov cx, 0x100 ; 256 words
+  rep stosw
+  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
+
+malloc_mark_allocated:
+  call malloc_calc_offsets
+  or byte [bx+di], al
+  ret
+
+malloc_mark_free:
+  call malloc_calc_offsets
+  not al
+  and byte [bx+di], al
+  ret