kernel: relocate to HMA if available

kernel/hma.asm

@@ -0,0 +1,32 @@
+; Code related to memory segment from 0xFFFF0 - 0x10FFEF
+
+; Check if 0000:0000 and FFFF:0010 point to the same memory
+; ZF=1 when A20 disabled and HMA unusable
+; ZF=0 when A20 enabled
+a20_enabled:
+  mov ax, 0x0000
+  mov ds, ax
+  mov si, ax
+  mov ax, 0xFFFF
+  mov di, 0x0010
+  mov cx, 0x0200
+  repe cmpsw
+  ret
+
+; Relocate kernel code to HMA
+hma_relocate:
+  call a20_enabled
+  ; TODO: talk to hw to enable a20
+  jz .end
+  mov ax, cs
+  mov ds, ax
+  mov si, 0x0010
+  mov ax, 0xFFFF
+  mov es, ax ; new code segment
+  mov ss, ax ; new stack segment
+  mov di, 0x0010
+  mov cx, 0x7FF8 ; 0xFFF0 divided by 2
+  repe movsw
+  jmp 0xFFFF:.end
+.end:
+  ret

kernel/intr.asm

@@ -14,10 +14,10 @@ ivt_backup:
 
 ; set item in interrupt vector table
 ; in:	bx	interrupt number
-;	ds:dx	new handler address
+;	cs:dx	new handler address
 ivt_set:
-  push es
   push ax
+  push es
 
   xor ax,ax
   mov es,ax
@@ -28,6 +28,6 @@ ivt_set:
   mov [es:bx], dx
   mov [es:bx+2], cs
 
-  pop ax
   pop es
+  pop ax
   ret

kernel/main.asm

@@ -4,55 +4,27 @@ org 0x0000
 
 _startup:
   mov ax, cs
-  mov ds, ax
-  mov es, ax
   mov ss, ax
   mov sp, 0x0000
+  ; <0x0010 is BIOS segment when relocated to HMA
+  ; so we need a nop sled
+  times (0x10 - ($-$$)) nop
 main:
-  xor ax, ax
-  mov ds, ax
-  mov si, ax
-  mov cx, ax
-
-  call debug_init
-  int 0x2E
-
+  call hma_relocate
   call ivt_backup
+  call debug_init
 
-  xor ax, ax
-  mov ds, ax
-  mov es, ax
-  xor cx, cx
-  mov si, ax
-.loop:
-  mov ax, cx
-  call kprint8
-  mov al, '='
-  call kputc
-  lodsw
-  xchg ax, dx
-  lodsw
+  int 0x2E
+  mov ax,0x1234
+  int 0x2E
   call kprint16
-  mov al, ':'
-  call kputc
-  xchg ax, dx
-  call kprint16
-  mov al, ' '
-  call kputc
-  inc cx
-  test cx, 0x0003
-  jnz .loop
-  mov al, 0x0A
-  call kputc
-  mov al, 0x0D
-  call kputc
-  cmp cx, 0x0060
-  jl .loop
 
 .halt:
   hlt
   jmp .halt
 
+%include "hma.asm"
+
 %include "heap.asm"
 %include "intr.asm"
 %include "debug.asm"