implement functionality of RAMTAS routine as described in C64 Programmer's Reference Guide

c64/kernal/ramtas.s

@@ -1,3 +1,38 @@
 ; Function defined on pp272-273 of C64 Programmers Reference Guide
 ramtas:
+	; C64 Programmer's Reference guide p291:
+
+	; Clear $0000-$0101, $0200-$03ff
+	; PGS: $0000, $0001 are CPU IO ports, so shouldn't get written to
+	LDY #$02
+	LDA #$00
+ramtas_l1:
+	STA $02,Y
+	INY
+	BNE ramtas_l1
+ramtas_l2:
+	STA $0200,Y
+	STA $0300,Y
+	INY
+	BNE ramtas_l2
+
+	; allocate cassette buffer
+
+	; set screen address to $0400
+	; https://www.c64-wiki.com/wiki/Screen_RAM
+	; Since we are setting $D018, we also make sure we are using the 
+	; ROM character set (C64 Programmer's Reference Guide p322)
+	LDA #$14
+	STA $D018
+
+	; Make sure we have the correct VIC-II memory bank for the screen to really be at $0400
+	; and not $4400, $8400 or $C400
+	; https://www.c64-wiki.com/wiki/VIC_bank#Selecting_VIC_banks
+	LDA $DD02
+	ORA #$03
+	STA $DD02
+	LDA $DD00
+	ORA #$03
+	STA $DD00
+	
 	rts