enable a20

boot1.s

@@ -223,22 +223,25 @@ main:
   call vga_println
 
   call test_a20
-  test ax, ax
+  test al, al
   jnz .a20_enabled
   
   mov ax, msg_a20_disabled
   call vga_println
 
-  mov eax, 0xa1b2c3d4
-  mov ebx, 0x12345678
-  mov ecx, 0xdeadbeef
-  mov edx, 0xcafebabe
-  mov esi, 0xfeedface
-  mov edi, 0x66778899
-  panic PANIC_TYPE_A20
+  mov ax, msg_a20_8042
+  call vga_println
 
-  ; TODO: enable a20
-  hlt
+  ; Try to enable A20 using the Intel 8042 PS/2 keyboard controller.
+  call enable_a20_intel_8042
+  call test_a20
+  test al, al
+  jnz .a20_enabled
+
+  ; TODO: try other methods first before we panic:
+  ; - [ ] BIOS interrupt
+  ; - [ ] Fast A20 enable
+  panic PANIC_TYPE_A20
 
 .a20_enabled:
   mov ax, msg_a20_enabled
@@ -596,11 +599,6 @@ nybble_to_hex_char:
   ret
 
 
-enable_a20:
-  fnstart
-  fnret
-
-
 ; Check whether the A20 line is enabled. Writes to the boot sector identifier.
 ; Arguments: none
 ; Return:
@@ -638,9 +636,78 @@ test_a20:
   ret
 
 
+; Wait for the Intel 8042 input buffer to become empty, so we can write.
+; Arguments: none
+; Return: none
+; Clobber: al
+intel_8042_wait_write:
+.loop:
+  ; Read the 8042 status register.
+  in al, INTEL_8042_IN_STATUS
+  ; Input buffer status flag set means the input buffer is full, so loop in this case.
+  test al, (1 << INTEL_8042_STATUS_IBUF)
+  jnz .loop
+  ret
+
+
+; Wait for the Intel 8042 output buffer to become filled, so we can read.
+; Arguments: none
+; Return: none
+; Clobber: al
+intel_8042_wait_read:
+.loop:
+  ; Read the 8042 status register.
+  in al, INTEL_8042_IN_STATUS
+  ; Output buffer status flag unset means output buffer is empty, so loop in this case.
+  test al, (1 << INTEL_8042_STATUS_OBUF)
+  jz .loop
+  ret
+
+
+; Try to enable A20 using the Intel 8042 PS/2 keyboard controller.
+; Arguments: none
+; Return: none
+; Clobber: ax, cx, dx
+enable_a20_intel_8042:
+  ; Temporarily disable the keyboard.
+  call intel_8042_wait_write
+  mov al, INTEL_8042_CMD_PS2_1_DISABLE
+  out INTEL_8042_OUT_CMD, al
+
+  ; Read the controller output port.
+  call intel_8042_wait_write
+  mov al, INTEL_8042_CMD_CONTROLLER_OUT_PORT_READ
+  out INTEL_8042_OUT_CMD, al
+  call intel_8042_wait_read
+  in al, INTEL_8042_IO_DATA
+
+  ; The second bit is "A20 enabled", so set it.
+  mov cl, al
+  or cl, 2
+
+  ; Write the modified byte back to the controller output port.
+  call intel_8042_wait_write
+  mov al, INTEL_8042_CMD_CONTROLLER_OUT_PORT_WRITE
+  out INTEL_8042_OUT_CMD, al
+  call intel_8042_wait_write
+  mov al, cl
+  out INTEL_8042_IO_DATA, al
+
+  ; Re-enable the keyboard.
+  call intel_8042_wait_write
+  mov al, INTEL_8042_CMD_PS2_1_ENABLE
+  out INTEL_8042_OUT_CMD, al 
+
+  ; Wait for writes to finish.
+  call intel_8042_wait_write
+
+  ret
+
+
 msg_boot1_loaded db "boot1 loaded. hello!", 0
 msg_a20_enabled  db "a20 enabled", 0
 msg_a20_disabled db "a20 not enabled", 0
+msg_a20_8042     db "trying 8042", 0
 msg_panic        db "panic!", 0
 
 msg_reg_eip   db "eip", 0