Add BootROM support.

XTMax/Drivers/BootROM/bootrom.asm

@@ -0,0 +1,72 @@
+bits 16
+
+%ifndef AS_COM
+;
+; BIOS will look for the AA55 signature between C8000-DFFFF in 2KB increments.
+; We choose an address in that range.
+;
+org 0xce000
+dw 0AA55h
+db 4    ; times 512 bytes
+
+%else
+;
+; Building as a COM file for testing.
+;
+org 0x100
+
+%endif
+
+entry:
+%ifndef AS_COM
+    mov ax, 0xc000
+    mov ds, ax
+    mov es, ax
+%endif
+
+    mov si, welcome
+    call print_string
+
+
+%ifndef AS_COM
+    retf
+%else
+;
+; DOS exit program.
+;
+    mov ah, 0x4c
+    xor al, al
+    int 0x21
+%endif
+
+;
+; Utilities
+;
+
+print_string:
+    push bx
+    xor bx,bx
+.loop:
+    lodsb
+    or al, al
+    jz .done
+    mov ah, 0x0E
+    int 0x10
+    jmp .loop
+.done:
+    pop bx
+    ret
+
+;
+; Strings
+;
+
+welcome db 'BootROM for XTMax v0.1', 0x0D, 0x0A, 0
+
+%ifndef AS_COM
+;
+; Pad to 2KB. We will try to keep our ROM under that size.
+;
+times 2047-($-$$) db 0
+db 0    ; will be used to complete the checksum.
+%endif