;; Generic crt0.s for a Z80
        .module crt0
       	.globl	_main
	.globl _line_num
	.globl _cur_pos
	.globl	calc_screen_addr
	
	.area	_HEADER (ABS)
	;; Reset vector
	.org 	0
	jp	init

	.org	0x08
	jr	put_char
	.org	0x10
	reti
	.org	0x18
	reti
	.org	0x20
	reti
	.org	0x28
	reti
	.org	0x30
	reti
	.org	0x38
	reti

	.org	0x51		; normal ROM happens to have ei ret here too 

EIROM:	ei			; for starting SNA images
	ret
	
	
put_char:
	cp	#1
	ret	nz
	ld	a,l
	cp	#0x0d		;  ignore CR
	ret	z
	cp	#0x0A
	ret	z
	push	bc
	push	hl		; l has the char

	;; 	call	calc_screen_addr
	call	calc_screen_addr_apple
	
	pop	bc		; c has the char - this used to be HL		
	ld	(hl),c

	pop	bc
	ret


	;; calculate the screen address for the current character

	;; almost like Apple but added 0x4000 as 0x400 is code in M4K
calc_screen_addr_apple:
	ld	a,(#_line_num)	; 0 to $17
	;; 	rrca			;  000ABCDE -> high 1CD, low EABAB000
	and	#7
	or	#0x88		; will get to be 44
	ld	h,a		; high byte 44 to 47

	ld	a,(#_line_num)
	and	#0x18		;  e00ab000
	ld	l,a
	sla	l
	sla	l
	or	l		; a should have 0abab000
	rla			; a should have abab000x
	
	sra	h		; carry gets E
	res	7,h
	rra			; a should have Eabab000
	ld	l,a		; l should have eabab000

	ld	a,  (_cur_pos)
	ld	c,a	
	inc	a
	ld	(_cur_pos),a
	xor	a
	ld	b,a
	add	hl, bc
	ret

	
	;; destroys hl,a,bc
calc_screen_addr:	
	
	xor	a
	ld	h,a
	ld	a,(#_line_num)
	ld	l,a
	add	hl,hl 		; *2
	add	hl,hl 		; *4
	add	hl,hl 		; *8
	add	hl,hl 		; *16	
	add	hl,hl 		; *32
	add	hl,hl 		; *64				
	
	ld	bc,#0x4000
	add	hl,bc		;  start of curr line
add_offset:		
	ld	a, (_cur_pos)
	push	af
	inc	a
	ld	(_cur_pos),a
	pop	af
	add	a,l
	jr	nc, no_carry
	inc	h
no_carry:
	ld	l,a		;  now we have the final address for a char
	ret		

	.globl _key_pressed

_key_pressed:
	in	a,(0) 		; get ascii
	ld	l,a	
	ret
	
;;	.org	0x100

	        .area   _CODE
init:
	;; Stack at the top of memory.
	ld	sp,#0xfffe

        ;; Initialise global variables
	call    gsinit
	call	_main
	jp	_exit

	;; Ordering of segments for the linker.
	.area	_HOME
	.area	_CODE
        .area   _GSINIT
        .area   _GSFINAL

	.area	_DATA
        .area   _BSS
        .area   _HEAP

        .area   _CODE
__clock::
	ld	a,#2
        rst     0x08
	ret

_exit::
	;; Exit - special code to the emulator
	ld	a,#0
        rst     0x08
1$:
	halt
	jr	1$

        .area   _GSINIT
gsinit::

        .area   _GSFINAL
        ret