started work on setting text mode

src/kernal/screen_m65/mega65_clrscr.s

@@ -0,0 +1,9 @@
+// #LAYOUT# M65 KERNAL_1 #TAKE
+// #LAYOUT# *   *        #IGNORE
+
+
+M65_CLRSCR:
+
+	// XXX impplement this
+
+	rts

src/kernal/screen_m65/mega65_scrmodeget.s

@@ -0,0 +1,14 @@
+// #LAYOUT# M65 KERNAL_1 #TAKE
+// #LAYOUT# *   *        #IGNORE
+
+//
+// Output:
+// .A - current screen mode
+//
+
+
+M65_SCRMODEGET:
+
+	lda M65_SCRMODE
+
+	rts

src/kernal/screen_m65/mega65_scrmodeset.s

@@ -0,0 +1,99 @@
+// #LAYOUT# M65 KERNAL_1 #TAKE
+// #LAYOUT# *   *        #IGNORE
+
+//
+// Input:
+// - .A - desired screen mode
+//
+// Output:
+// - Carry set   = unsupported screen mode
+// - Carry clear = screen mode switched correctly
+//
+
+
+M65_SCRMODESET:
+
+	cmp #$03
+	bcs m65_scrmodeset_rts              // branch if unsupported screen mode
+
+	// Preserve .X, move desired mode to .X, disable interrupts
+
+	phx
+	sei
+
+	// Perform the switch
+
+	jsr m65_scrmodeset_internal
+
+	// Re-enable interrupts, restore .X
+
+	cli
+	plx
+
+	// FALLTROUGH
+
+m65_scrmodeset_rts:
+
+	rts
+
+m65_scrmodeset_internal: // entry point for M65_MODE65
+
+	// Store desired screen mode
+
+	sta M65_SCRMODE
+
+	// Set the base screen addresses:
+	// - in VIC_SCRNPTR and M65_SCRTXTBASE to $16000
+	// - in VIC_COLPTR to $0000
+
+	ldx #$00
+	stx VIC_COLPTR+0
+	stx VIC_COLPTR+1
+	stx VIC_SCRNPTR+0
+	stx M65_SCRTXTBASE+0
+	stx VIC_SCRNPTR+3
+	stx M65_SCRTXTBASE+3
+	inx
+	stx VIC_SCRNPTR+2
+	stx M65_SCRTXTBASE+2
+	ldx #$06
+	stx VIC_SCRNPTR+1
+	stx M65_SCRTXTBASE+1
+
+	// Set flags for 40/80 columns and 25/50 rows
+
+	asl
+	tax
+	lda VIC_SCFLAGS
+	jsr_ind_x m65_scrmodeset_flagtab
+
+	rts
+
+
+
+m65_scrmodeset_flagtab:
+
+	.word m65_scrmodeset_40x25
+	.word m65_scrmodeset_80x25
+	.word m65_scrmodeset_80x50
+
+
+m65_scrmodeset_40x25:
+
+	and #$77
+	bra !+
+
+m65_scrmodeset_80x25:
+
+	and #$F7
+	ora #$80
+	skip_2_bytes_trash_nvz
+
+	// FALLTROUGH
+
+m65_scrmodeset_80x50:
+
+	ora #$88
+!:
+	sta VIC_SCFLAGS
+	rts