symon/samples/ehbasic_crtc/min_mon.asm

; minimal monitor for EhBASIC and 6502 simulator V1.05

; To run EhBASIC on the simulator load and assemble [F7] this file, start the simulator
; running [F6] then start the code with the RESET [CTRL][SHIFT]R. Just selecting RUN
; will do nothing, you'll still have to do a reset to run the code.

	.feature labels_without_colons
	.include "basic.asm"

; put the IRQ and MNI code in RAM so that it can be changed

IRQ_vec	= VEC_SV+2		; IRQ code vector
NMI_vec	= IRQ_vec+$0A	; NMI code vector

; setup for the 6502 simulator environment

IO_AREA		= $8800
ACIAdata	= IO_AREA		; simulated ACIA r/w port
ACIAstatus	= IO_AREA+1
ACIAcommand	= IO_AREA+2
ACIAcontrol	= IO_AREA+3

	CADDR_L	= $E0		; Cursor address (low)
	CADDR_H	= $E1		; Cursor address (high)
	C_COL	= $E2		; Cursor column
	C_ROW	= $E3		; Cursor row
	COUTC	= $E4		; Temp storage for char out.


; now the code. all this does is set up the vectors and interrupt code
; and wait for the user to select [C]old or [W]arm start. nothing else
; fits in less than 128 bytes

.segment "MONITOR"
	.org	$FC00			; pretend this is in a 1/8K ROM

; reset vector points here

RES_vec
	CLD				; clear decimal mode
	LDX	#$FF			; empty stack
	TXS				; set the stack

	;; Initialize the CRTC
	LDA	#$00		; Set cursor start to $7000
	STA	CADDR_L
	LDA	#$70
	STA	CADDR_H
	;; TODO: Initialize params on CRTC

; Initialize the ACIA
ACIA_init
	LDA	#$00
	STA	ACIAstatus		; Soft reset
	LDA	#$0B
	STA	ACIAcommand		; Parity disabled, IRQ disabled
	LDA	#$1E
	STA	ACIAcontrol		; Set output for 8-N-1 9600

; set up vectors and interrupt code, copy them to page 2

	LDY	#END_CODE-LAB_vec	; set index/count
LAB_stlp
	LDA	LAB_vec-1,Y		; get byte from interrupt code
	STA	VEC_IN-1,Y		; save to RAM
	DEY				; decrement index/count
	BNE	LAB_stlp		; loop if more to do

; now do the signon message, Y = $00 here

LAB_signon
	LDA	LAB_mess,Y		; get byte from sign on message
	BEQ	LAB_nokey		; exit loop if done

	JSR	V_OUTP		; output character
	INY				; increment index
	BNE	LAB_signon		; loop, branch always

LAB_nokey
	JSR	V_INPT		; call scan input device
	BCC	LAB_nokey		; loop if no key

	AND	#$DF			; mask xx0x xxxx, ensure upper case
	CMP	#'W'			; compare with [W]arm start
	BEQ	LAB_dowarm		; branch if [W]arm start

	CMP	#'C'			; compare with [C]old start
	BNE	RES_vec		; loop if not [C]old start

	JMP	LAB_COLD		; do EhBASIC cold start

LAB_dowarm
	JMP	LAB_WARM		; do EhBASIC warm start


ACIAout:

	PHA
@loop:	LDA	ACIAstatus
	AND	#$10
	BEQ	@loop		; Wait for buffer to empty
	PLA
	STA	ACIAdata
	RTS

;;; Byte out to the CRTC
;;;
;;; 1. Increment cursor position.
;;; 2. Scroll if necessary.
;;; 3. Store new cursor position in CRTC.

CRTCout:
	STA	COUTC		; Store the character going out
	JSR	ACIAout		; Also echo to terminal for debugging.

	;; In parallel, we're maintaining two states:
	;;    - The address of the cursor, and
	;;    - The Column/Row of the cursor.
	;;
	;; The latter state is used to handle scrolling and
	;; for knowing how far to back up the cursor
	;; for a carriage-return.

	;; Backspace
	CMP	#$08
	BEQ	DO_BS
	;; Line Feed
	CMP	#$0a
	BEQ	DO_BS
	;; Carriage Return
	CMP	#$0d
	BEQ	DO_CR
	;; Any other character	
	RTS

DO_BS:	RTS
DO_LF:	RTS
DO_CR:	RTS

	;; Handle a scroll request
DO_SCROLL:
	RTS
	
	;; Decrement the cursor address
INC_CADDR:
	INC	CADDR_L
	BNE	@l1		; Did we increment to 0?
	INC	CADDR_H		; Yes, also increment high
@l1	RTS  

	;; Increment the cursor address
DEC_CADDR:
	CMP	CADDR_L		; Is low alrady 0?
	BNE	@l1
	DEC	CADDR_H		; Yes, decrement high
@l1	DEC	CADDR_L
	RTS


;
; byte in from ACIA. This subroutine will also force
; all lowercase letters to be uppercase.
;
ACIAin
	LDA	ACIAstatus		; Read 6551 status
	AND	#$08			;
	BEQ	LAB_nobyw		; If rx buffer empty, no byte

	LDA	ACIAdata		; Read byte from 6551
	CMP	#'a'			; Is it < 'a'?
	BCC	@done			; Yes, we're done
	CMP	#'{'			; Is it >= '{'?
	BCS	@done			; Yes, we're done
	AND	#$5f			; Otherwise, mask to uppercase
@done
	SEC				; Flag byte received
	RTS

LAB_nobyw
	CLC				; flag no byte received
no_load				; empty load vector for EhBASIC
no_save				; empty save vector for EhBASIC
	RTS

; vector tables

LAB_vec
	.word	ACIAin		; byte in from simulated ACIA
	.word	CRTCout		; byte out to simulated ACIA
	.word	no_load		; null load vector for EhBASIC
	.word	no_save		; null save vector for EhBASIC

; EhBASIC IRQ support

IRQ_CODE
	PHA				; save A
	LDA	IrqBase		; get the IRQ flag byte
	LSR				; shift the set b7 to b6, and on down ...
	ORA	IrqBase		; OR the original back in
	STA	IrqBase		; save the new IRQ flag byte
	PLA				; restore A
	RTI

; EhBASIC NMI support

NMI_CODE
	PHA				; save A
	LDA	NmiBase		; get the NMI flag byte
	LSR				; shift the set b7 to b6, and on down ...
	ORA	NmiBase		; OR the original back in
	STA	NmiBase		; save the new NMI flag byte
	PLA				; restore A
	RTI

END_CODE

; sign on string

LAB_mess
	.byte	$0D,$0A,"Symon (c) 2008-2014, Seth Morabito"
	.byte   $0D,$0A,"Enhanced 6502 BASIC 2.22 (c) Lee Davison"
	.byte   $0D,$0A,"[C]old/[W]arm ?",$00


; system vectors

.segment "VECTORS"
	.org	$FFFA

	.word	NMI_vec		; NMI vector
	.word	RES_vec		; RESET vector
	.word	IRQ_vec		; IRQ vector