2020/c64/screen.asm

// vim: filetype=kickass

//
// clear_screen
//
// Destroys: a, y
//
clear_screen:
    ldy #0

!loop:
    lda #' '
    sta SCREEN_RAM, y
    sta SCREEN_RAM + $0100, y
    sta SCREEN_RAM + $0200, y
    sta SCREEN_RAM + $0300, y
    sta SCREEN_RAM + $03e7, y

    lda #1
    sta COLOR_RAM, y
    sta COLOR_RAM + $0100, y
    sta COLOR_RAM + $0200, y
    sta COLOR_RAM + $0300, y
    sta COLOR_RAM + $03e7, y

    iny
    bne !loop-

    lda #<SCREEN_RAM
    sta cursor_pointer_lo
    lda #>SCREEN_RAM
    sta cursor_pointer_hi

    lda #23
    sta $d018

    rts


//
// print_carriage_return
//
// Input:
//  - cursor_pointer_{lo,hi} - Cursor pointer.
//
// Destroys: a, zp_temp
//
print_newline:
    sec
    lda $05
    adc #40
    sta cursor_pointer_lo
    bcc !done+
    inc cursor_pointer_hi

!done:
    rts


//
// print_carriage_return
//
// Input:
//  - cursor_pointer_{lo,hi} - Cursor pointer.
//
// Destroys: a, zp_temp
//
print_carriage_return:
    // Subtract SCREEN_RAM from cursor pointer
    sec
    lda cursor_pointer_hi
    sbc #>SCREEN_RAM
    sta cursor_pointer_hi

    // Divide the cursor position by 40 (one row)
    i16_div8(cursor_pointer_lo, cursor_pointer_hi)

    // At this point, the pointer fits in a 8-bit register
    lda cursor_pointer_lo
    i8_div5_a()

    // Multiply it by 40 again
    i8_mul5_a()
    sta cursor_pointer_lo

    i16_mul8(cursor_pointer_lo, cursor_pointer_hi)

    // Add SCREEN_RAM back
    clc
    lda cursor_pointer_hi
    adc #>SCREEN_RAM
    sta cursor_pointer_hi

    rts


//
// write_string
//
// Input:
//  - $03-$04 - Pointer to null-terminated string.
//  - $05-$06 - Cursor pointer.
//
// Destroys: a, x, y
//
write_string:
    ldx #0
    ldy #0

!loop:
    lda ($03), y
    beq !done+

    cmp #'\r'
    beq !carriage_return+
    cmp #'\n'
    beq !newline+

    sta (cursor_pointer_lo, x)

    i16_inc(cursor_pointer_lo, cursor_pointer_hi)

    iny
    jmp !loop-

!newline:
    jsr print_newline

!carriage_return:
    jsr print_carriage_return

    iny
    jmp !loop-

!done:
    rts


print_hex_0x:
    sta zp_temp

    ldx #0

    lda #'0'
    sta (cursor_pointer_lo, x)
    i16_inc(cursor_pointer_lo, cursor_pointer_hi)

    lda #'x'
    sta (cursor_pointer_lo, x)
    i16_inc(cursor_pointer_lo, cursor_pointer_hi)

    lda zp_temp

    // fallthrough

print_hex:
    ldx #0
    sta zp_temp

    lsr
    lsr
    lsr
    lsr
    tay

    lda !hex_chars+, y
    sta (cursor_pointer_lo, x)
    i16_inc(cursor_pointer_lo, cursor_pointer_hi)

    lda zp_temp
    and #$0f
    tay

    lda !hex_chars+, y
    sta (cursor_pointer_lo, x)
    i16_inc(cursor_pointer_lo, cursor_pointer_hi)

    rts

!hex_chars:
    .text "0123456789ABCDEF"