Check for userprog download on a timer rather than explicitly

host/src/STARTUP.s

@@ -69,15 +69,19 @@
 
         ;; Disable maskeble interups (not all)
         SEI
-        ;; Disable BASIC ROM mohahaha
+        ;; Set up bank switching.
+	;; Disable BASIC and KERNAL(sic) ROM, keep I/O
 	;; https://www.c64-wiki.com/wiki/Bank_Switching
-        LDA #%11111110
-        AND $0001
-        STA $0001
+        LDA #%11111000
+        AND $01
+        STA $01
+	LDA #%00000101
+	ORA $01
+	STA $01
 
         ;;Disable non maskable interupts
         ;;https://codebase64.org/doku.php?id=base:nmi_lock_without_kernal
-        ;; write to $FFFA/$FFFB possible (and needed) if BASIC ROM is disabled
+        ;; write to $FFFA/$FFFB possible (and needed) if KERNAL ROM is disabled
 
         LDA #<NMI_routine
         STA $FFFA
@@ -92,8 +96,14 @@
         LDA #$01
         STA $DD0E ;; start Timer A -> NMI
         ;; from here on NMI is disabled
-        JMP NMI_routine_end
+
+	;; Set up IRQ for userprog transfer timer
+	LDA #<IRQ_routine
+	STA $FFFE
+	LDA #>IRQ_routine
+	STA $FFFF
+
+        JMP mainsetup
 NMI_routine:
         RTI ;; exit interrupt not acknowledged
-NMI_routine_end:
 .endscope

host/src/mainloop.s

@@ -1,4 +1,5 @@
 ;;; -*- Mode: asm; indent-tabs-mode: t; tab-width: 8 -*-
+mainsetup:
 .scope mainsetup
 .ifdef EMULATOR_PREVIEW
  	USERPROG	=	$C000
@@ -20,6 +21,7 @@
 	bne @waitstart
 
 	jsr setup_loader
+	jmp endsetup
 
 	pha
 @trying:
@@ -33,19 +35,42 @@
 	jsr maybe_download_userprog
 	beq @trying
 .endif
-.endscope
 
+endsetup:
+	DOWNLOAD_TIMER	=	$FFFF
 	lda #$0d
 	sta $d020
 
-.scope mainloop
+	;; Turn on CIA 1's Timer A as IRQ source Summary: Timer A counts down
+	;; for 0xFFFF cycles. When it finishes, an IRQ is sent to the
+	;; CPU. Once it's ready to receive it, the IRQ routine is called,
+	;; which checks if a userprog download is beginning. If it is, we
+	;; download and reset the unwind the stack. The transfer hardware has
+	;; a certain delay between the transfer command being sent and the
+	;; start of bits being sent. This duration must be some amount longer
+	;; than the timer's duration to account for variable execution time of
+	;; interrupt-free routines.
+
+	sei 			; let's be extra sure interrupts are disabled
+
+	lda #<DOWNLOAD_TIMER
+	sta $dc04		; Timer A's counter low byte
+	lda #>DOWNLOAD_TIMER
+	sta $dc05		; Likewise for high byte
+
+	lda #%10000001		; Send IRQ to CPU when Timer A is done
+	sta $dc0d
+
+	lda #%00011001		; Start Timer A, one-shot mode, cycle counting mode
+	sta $dc0e
 
 	ldx #$00
 	stx FRAMECOUNT
 	inx
 	sta FIRSTTIME
+.endscope
 
-ml:
+.proc mainloop
 	USERPROG	=	$C000
 	RASTER		=	$D012
 
@@ -54,21 +79,9 @@ ml:
 	ora $01
 	sta $01
 
-.ifndef EMULATOR_PREVIEW
-	jsr maybe_download_userprog
-	beq @nochange
-	ldx #$00
-	stx FRAMECOUNT
-	inx
-	sta FIRSTTIME
-@nochange:
-.endif
+	cli
 	jsr USERPROG
 
-	;; Bank out character ROM, I/O in
-	lda #%00000100
-	ora $01
-	sta $01
 @framewait:
 	ldy RASTER
 	cpy #$66
@@ -76,8 +89,8 @@ ml:
 	inc FRAMECOUNT
 	lda #$00
 	sta FIRSTTIME
-	jmp ml
-.endscope
+	jmp mainloop
+.endproc
 
 .proc time			; user-procedure
 	lda FRAMECOUNT
@@ -101,6 +114,41 @@ ml:
 	rts
 .endproc
 
+.proc reset_userprog
+	ldx #$00
+	stx FRAMECOUNT
+	inx
+	sta FIRSTTIME
+.endproc
 
 FRAMECOUNT:	.byte 0
 FIRSTTIME:	.byte 1
+
+IRQ_routine:
+	lda $dc0d		; Check CIA 1's status
+	beq @somethingelse
+	jsr maybe_download_userprog
+	beq @done
+	jsr reset_userprog
+
+	;; Reconstruct the stack to exit to toplevel
+	ldx #$00
+	txs
+	lda #<mainloop
+	pha
+	lda #>mainloop
+	pha
+	txa
+	pha
+
+	lda #%00011001		; Start Timer A, one-shot mode, cycle counting
+	sta $dc0e
+
+	rti
+
+@somethingelse:
+	;; TODO: Ack potential VIC-II IRQ
+@done:
+	lda #%00011001		; Start Timer A, one-shot mode, cycle counting
+	sta $dc0e
+	rti