more work on RAMTAS routine based on Compute's Mapping the 64

c64/kernal/ramtas.s

@@ -1,7 +1,6 @@
 ; 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
@@ -17,6 +16,11 @@ ramtas_l2:
 	BNE ramtas_l2
 
 	; allocate cassette buffer
+	;; "Mapping the C128", p61
+	lda #<$033C
+	STA cassette_buffer_ptr+0
+	lda #>$033C
+	STA cassette_buffer_ptr+1
 
 	; set screen address to $0400
 	; https://www.c64-wiki.com/wiki/Screen_RAM
@@ -34,5 +38,33 @@ ramtas_l2:
 	LDA $DD00
 	ORA #$03
 	STA $DD00
+
+	;; Set screen address pointer ("Compute's Mapping the 64" p238)
+	LDA #>$0400
+	STA HIBASE
+	
+	;;  Work out RAM size and put in MEMSTR and MEMSIZ
+	;; "Compute's Mapping the 64", p54
+	lda #$00
+	sta MEMSTR+0
+	sta MEMSIZ+0
+	;; Try to modify $8000, if it fails then RAM ends at $7FFF, else $9FFF
+	lda $8000
+	eor #$FF
+	tax
+	eor #$FF
+	stx $8000
+	cmp $8000
+	bne ramtas_32K_RAM
+	sta $8000
+	lda #$A0
+	sta MEMSIZ+1
+	jmp ramtas_RAM_sized
+ramtas_32K_RAM:	
+	sta $8000 		; in case RAM is hiding beneath a ROM
+	lda #$80
+	sta MEMSIZ+1
+	;; Fall through
+ramtas_RAM_sized:	
 	
 	rts