//
// Official Kernal routine, described in:
//
// - [RG64] C64 Programmer's Reference Guide   - page 291
// - [CM64] Compute's Mapping the Commodore 64 - page 237
//
// CPU registers that has to be preserved (see [RG64]): none
//

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 $00,Y
	iny
	bne ramtas_l1
ramtas_l2:
	// How many ways are there to efficiently erase these two pages
	// of RAM? We would like to avoid any unnecessary byte similarity
	// with the C64 KERNAL. Thus we do $0300 before $0200, even though
	// with the longer sequence of identicale bytes we see no basis for
	// it being copyrightable.  Again, we just want to redue the attack
	// surface for any misguided suit.
	sta $0300,Y
	sta $0200,Y
	iny
	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
	// 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 VIC_YMCSB

	// 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 CIA2_DDRA
	ora #$03
	sta CIA2_DDRA
	lda CIA2_PRA
	ora #$03
	sta CIA2_PRA

	// Set screen address pointer ("Compute's Mapping the 64" p238)
	// This is obvious boiler plate containing no creative input, but to avoid
	// unnecessarily similarity to the C64 KERNAL, we use X instead of A to do this,
	// and several following
	ldx #>$0400
	stx HIBASE
	
	//  Work out RAM size and put in MEMSTR and MEMEND
	// "Compute's Mapping the 64", p54
	//  http://unusedino.de/ec64/technical/project64/mapping_c64.html
	ldx #>$0800
	stx MEMSTR+1
	ldx #$00
	stx MEMSTR+0
	stx MEMEND+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
	beq ramtas_32K_RAM
	sta $8000
	ldx #$A0
	stx MEMEND+1
	jmp ramtas_RAM_sized
ramtas_32K_RAM:	
	sta $8000 		// in case RAM is hiding beneath a ROM
	ldx #$80
	stx MEMEND+1
	// Fall through
ramtas_RAM_sized:	
	
	rts