path: root/src/term.asm

; =============================================================================
; term.asm - a 40x18 text terminal on the CGB LCD, with cursor and fast scroll.
;
; 4x8 font: each 8x8 background tile holds TWO 4-px characters (left+right),
; giving 40 columns. Tiles are LINE-BOUND: line-slot L owns the 20 VRAM tiles
; at tile-positions [L*20 .. L*20+19] (index = LOW(pos), VRAM bank = HIGH(pos);
; positions 256-359 live in bank 1 - hence CGB-only). assign[sr] maps each of
; the 18 screen rows to a line-slot, and the tilemap points each screen row at
; that slot's tiles. Scrolling just rotates assign[], rebuilds the one new
; bottom line, and rewrites the tilemap - O(1 line), not O(screen).
;
; The cursor is an underline OR'd into the current cell's tile (wCurMask).
; =============================================================================
INCLUDE "include/gbos.inc"

DEF rSCY    EQU $FF42
DEF rSCX    EQU $FF43
DEF rVBK    EQU $FF4F   ; CGB VRAM bank select
DEF rBCPS   EQU $FF68   ; CGB BG palette index
DEF rBCPD   EQU $FF69   ; CGB BG palette data

DEF TCOLS   EQU 40      ; text columns
DEF TROWS   EQU 18      ; text rows
DEF TSTRIDE EQU 64      ; bytes per line-slot in the buffer (power of 2)
DEF TERM_BUF EQU $D600  ; 18*64 = 1152 B (WRAMX, above the FS caches)

; terminal state (WRAMX gap $D580-$D5FF)
DEF wGlyphL   EQU $D580  ; 2
DEF wGlyphR   EQU $D582  ; 2
DEF wVram     EQU $D584  ; 2
DEF wCurRow   EQU $D586  ; cursor screen row 0..17
DEF wCurCol   EQU $D587  ; cursor screen col 0..39
DEF wCursorOn EQU $D588  ; draw the cursor?
DEF wRT_sr    EQU $D58A  ; render_tile scratch
DEF wRT_tcol  EQU $D58B
DEF wRT_cL    EQU $D58C
DEF wRT_cR    EQU $D58D
DEF wScratch  EQU $D58E
DEF wAssign   EQU $D590  ; assign[18]: screen row -> line-slot ($D590-$D5A1)

DEF CUR_L   EQU $E0      ; cursor underline, left cell (3px)
DEF CUR_R   EQU $0E      ; cursor underline, right cell

SECTION "term", ROM0

; -----------------------------------------------------------------------------
; term_init - CGB palette, identity assign[], clear buffer, build, LCD on.
; -----------------------------------------------------------------------------
term_init::
    xor a
    ld [rLCDC], a               ; LCD off
    ld [rSCX], a
    ld [rSCY], a

    ; BG palette 0: color0 = white (bg), color1 = black (text)
    ld a, $80
    ld [rBCPS], a
    ld a, $FF
    ld [rBCPD], a
    ld a, $7F
    ld [rBCPD], a
    xor a
    ld [rBCPD], a
    ld [rBCPD], a
    ld [rBCPD], a
    ld [rBCPD], a
    ld [rBCPD], a
    ld [rBCPD], a

    ; state: assign[sr]=sr, cursor at 0,0 (on)
    ld hl, wAssign
    ld b, TROWS
    xor a
.mkid
    ld [hl+], a
    inc a
    dec b
    jr nz, .mkid
    xor a
    ld [wCurRow], a
    ld [wCurCol], a
    ld a, 1
    ld [wCursorOn], a

    ; clear the buffer (all slots) to spaces
    ld hl, TERM_BUF
    ld de, TROWS * TSTRIDE
.clr
    ld a, $20
    ld [hl+], a
    dec de
    ld a, d
    or e
    jr nz, .clr

    call term_write_tilemap
    call term_redraw            ; build all tiles (blank + cursor)

    ld a, %10010001             ; LCD on, BG on, tile data $8000, map $9800
    ld [rLCDC], a
    ret

; -----------------------------------------------------------------------------
; term_write_tilemap - point each screen row's tilemap entries at assign[]'s
; line-slot tiles. Pass 1 = indices (bank 0), pass 2 = attributes (bank 1).
; -----------------------------------------------------------------------------
term_write_tilemap::
    xor a
    ld [rVBK], a
    ld c, 0                     ; screen row
.i_row
    call .slot20                ; DE = assign[c]*20 (pos start), HL = tilemap row
    ld b, TCOLS/2
.i_col
    ld a, e
    ld [hl+], a                 ; index = LOW(pos)
    inc de
    dec b
    jr nz, .i_col
    inc c
    ld a, c
    cp TROWS
    jr c, .i_row

    ld a, 1
    ld [rVBK], a
    ld c, 0
.a_row
    call .slot20
    ld b, TCOLS/2
.a_col
    ld a, d                     ; HIGH(pos): 0 or 1
    or a
    jr z, .a0
    ld a, $08                   ; VRAM bank 1 attribute
    jr .aw
.a0
    xor a
.aw
    ld [hl+], a
    inc de
    dec b
    jr nz, .a_col
    inc c
    ld a, c
    cp TROWS
    jr c, .a_row
    xor a
    ld [rVBK], a
    ret
; helper: C = screen row -> DE = assign[C]*20, HL = $9800 + C*32
.slot20
    ld a, c
    add LOW(wAssign)
    ld l, a
    ld a, HIGH(wAssign)
    adc 0
    ld h, a
    ld a, [hl]                  ; L = assign[row]
    ld l, a
    ld h, 0
    add hl, hl
    add hl, hl                  ; L*4
    ld d, h
    ld e, l
    add hl, hl
    add hl, hl                  ; L*16
    add hl, de                  ; L*20
    ld d, h
    ld e, l                     ; DE = pos start
    ld a, c
    ld l, a
    ld h, 0
    add hl, hl
    add hl, hl
    add hl, hl
    add hl, hl
    add hl, hl                  ; row*32
    ld a, h
    add $98
    ld h, a                     ; HL = $9800 + row*32
    ret

; -----------------------------------------------------------------------------
; slot_addr - A = screen row -> HL = TERM_BUF + assign[row]*64
; -----------------------------------------------------------------------------
slot_addr:
    add LOW(wAssign)
    ld l, a
    ld a, HIGH(wAssign)
    adc 0
    ld h, a
    ld a, [hl]                  ; line-slot
    ld l, a
    ld h, 0
    add hl, hl
    add hl, hl
    add hl, hl
    add hl, hl
    add hl, hl
    add hl, hl                  ; *64
    ld de, TERM_BUF
    add hl, de
    ret

; -----------------------------------------------------------------------------
; glyph_ptr - A = char -> HL = FontData + (clamp(A,32,127)-32)*8
; -----------------------------------------------------------------------------
glyph_ptr:
    cp 32
    jr nc, .ok
    ld a, 32
.ok
    cp 128
    jr c, .ok2
    ld a, 32
.ok2
    sub 32
    ld l, a
    ld h, 0
    add hl, hl
    add hl, hl
    add hl, hl                  ; *8
    ld de, FontData
    add hl, de
    ret

; -----------------------------------------------------------------------------
; build_tile - build tile for tile-position DE (E=index, D=bank) from chars
; B (left) and C (right). ORs wCurMask into the last pixel row (cursor).
; -----------------------------------------------------------------------------
build_tile::
    ld a, d
    ld [rVBK], a                ; VRAM bank for this tile
    ld h, 0
    ld l, e
    add hl, hl
    add hl, hl
    add hl, hl
    add hl, hl                  ; E*16
    ld a, h
    add $80
    ld h, a                     ; HL = $8000 + E*16
    ld a, l
    ld [wVram], a
    ld a, h
    ld [wVram+1], a
    ld a, b
    call glyph_ptr
    ld a, l
    ld [wGlyphL], a
    ld a, h
    ld [wGlyphL+1], a
    ld a, c
    call glyph_ptr
    ld a, l
    ld [wGlyphR], a
    ld a, h
    ld [wGlyphR+1], a
    ld b, 8                     ; row counter
    ld a, [wVram]
    ld l, a
    ld a, [wVram+1]
    ld h, a                     ; HL = VRAM dest
.brow
    push hl
    ld a, [wGlyphL]
    ld e, a
    ld a, [wGlyphL+1]
    ld d, a
    ld a, [de]
    swap a
    and $F0
    ld c, a                     ; left glyph -> high nibble
    ld a, [wGlyphR]
    ld e, a
    ld a, [wGlyphR+1]
    ld d, a
    ld a, [de]
    and $0F
    or c                        ; A = combined pixel row
    ld e, a                     ; save (DE reloaded next iter)
    ld a, b
    dec a
    jr nz, .nocur               ; last row (b==1)? add cursor underline
    ld a, [wCurMask]
    or e
    jr .wr
.nocur
    ld a, e
.wr
    pop hl
    ld [hl+], a                 ; plane 0
    xor a
    ld [hl+], a                 ; plane 1 = 0
    ; advance glyph pointers - MUST NOT touch HL (it holds the VRAM dest!)
    ld a, [wGlyphL]
    inc a
    ld [wGlyphL], a
    jr nz, .nl
    ld a, [wGlyphL+1]
    inc a
    ld [wGlyphL+1], a
.nl
    ld a, [wGlyphR]
    inc a
    ld [wGlyphR], a
    jr nz, .nr
    ld a, [wGlyphR+1]
    inc a
    ld [wGlyphR+1], a
.nr
    dec b
    jr nz, .brow
    ret

; -----------------------------------------------------------------------------
; render_tile - B = screen row, C = tile column: rebuild that one tile from the
; buffer, adding the cursor underline if the cursor is in it.
; -----------------------------------------------------------------------------
render_tile:
    ld a, b
    ld [wRT_sr], a
    ld a, c
    ld [wRT_tcol], a
    ; chars: &buffer[assign[sr]][tcol*2]
    ld a, b
    call slot_addr              ; HL = &buffer[slot][0]
    ld a, [wRT_tcol]
    add a                       ; tcol*2
    add l
    ld l, a
    ld a, h
    adc 0
    ld h, a
    ld a, [hl+]
    ld [wRT_cL], a
    ld a, [hl]
    ld [wRT_cR], a
    ; cursor mask
    xor a
    ld [wCurMask], a
    ld a, [wCursorOn]
    or a
    jr z, .pos
    ld a, [wCurRow]
    ld b, a
    ld a, [wRT_sr]
    cp b
    jr nz, .pos
    ld a, [wRT_tcol]
    add a                       ; left cell col
    ld b, a
    ld a, [wCurCol]
    cp b
    jr nz, .cr
    ld a, CUR_L
    ld [wCurMask], a
    jr .pos
.cr
    inc b                       ; right cell col
    cp b
    jr nz, .pos
    ld a, CUR_R
    ld [wCurMask], a
.pos
    ; pos = assign[sr]*20 + tcol -> DE (D=bank, E=index)
    ld a, [wRT_sr]
    add LOW(wAssign)
    ld l, a
    ld a, HIGH(wAssign)
    adc 0
    ld h, a
    ld a, [hl]
    ld l, a
    ld h, 0
    add hl, hl
    add hl, hl
    ld d, h
    ld e, l                     ; L*4
    add hl, hl
    add hl, hl
    add hl, de                  ; L*20
    ld a, [wRT_tcol]
    add l
    ld l, a
    ld a, h
    adc 0
    ld h, a                     ; HL = pos
    ld d, h
    ld e, l                     ; DE = pos
    ld a, [wRT_cL]
    ld b, a
    ld a, [wRT_cR]
    ld c, a
    call build_tile
    xor a
    ld [rVBK], a
    ret

; render_cursor_tile - rebuild the tile at the cursor position.
render_cursor_tile:
    ld a, [wCurRow]
    ld b, a
    ld a, [wCurCol]
    srl a                       ; /2 = tile col
    ld c, a
    jp render_tile

; -----------------------------------------------------------------------------
; term_redraw - rebuild every tile from the buffer (init / full refresh).
; -----------------------------------------------------------------------------
term_redraw::
    ld b, 0                     ; screen row
.row
    ld c, 0                     ; tile col
.col
    push bc
    call render_tile
    pop bc
    inc c
    ld a, c
    cp TCOLS/2
    jr c, .col
    inc b
    ld a, b
    cp TROWS
    jr c, .row
    ret

; -----------------------------------------------------------------------------
; term_scroll - scroll up one line: rotate assign[], blank+build the new bottom
; line, rewrite the tilemap.
; -----------------------------------------------------------------------------
term_scroll::
    ld a, [wAssign]
    ld [wScratch], a            ; freed line-slot
    ld hl, wAssign+1
    ld de, wAssign
    ld b, TROWS-1
.rot
    ld a, [hl+]
    ld [de], a
    inc de
    dec b
    jr nz, .rot
    ld a, [wScratch]
    ld [wAssign + TROWS-1], a   ; new bottom row uses the freed slot
    ; clear that slot to spaces
    ld l, a
    ld h, 0
    add hl, hl
    add hl, hl
    add hl, hl
    add hl, hl
    add hl, hl
    add hl, hl                  ; *64
    ld de, TERM_BUF
    add hl, de
    ld b, TCOLS
.clr
    ld a, $20
    ld [hl+], a
    dec b
    jr nz, .clr
    ; rebuild bottom line tiles (screen row 17)
    ld c, 0
.build
    push bc
    ld b, TROWS-1
    call render_tile
    pop bc
    inc c
    ld a, c
    cp TCOLS/2
    jr c, .build
    call term_write_tilemap
    ret

; cursor_down - advance cursor row, scrolling if it runs off the bottom.
cursor_down:
    ld a, [wCurRow]
    inc a
    cp TROWS
    jr c, .ok
    call term_scroll
    ld a, TROWS-1
.ok
    ld [wCurRow], a
    ret

; -----------------------------------------------------------------------------
; term_putc - A = character. Print at the cursor, advance, handle \n \r \b.
; -----------------------------------------------------------------------------
; term_putc guarantees SVBK=1 so the buffer/state at $D5xx/$D6xx are the ones
; we built, no matter which process context KPutc calls us from. The stack is
; in non-banked WRAM ($Cxxx/$Axxx), so saving SVBK across the switch is safe.
term_putc::
    ld d, a
    ld a, [rSVBK]
    push af
    ld a, 1
    ld [rSVBK], a
    ld a, d
    call term_putc_body
    pop af
    ld [rSVBK], a
    ret
term_putc_body:
    cp 10
    jr z, .newline
    cp 13
    jr z, .cret
    cp 8
    jr z, .bs
    cp 32
    ret c                       ; ignore other control chars
    ; printable: buffer[assign[cur]][cc] = char
    ld [wRT_cL], a
    ld a, [wCurRow]
    call slot_addr
    ld a, [wCurCol]
    add l
    ld l, a
    ld a, h
    adc 0
    ld h, a
    ld a, [wRT_cL]
    ld [hl], a
    ; redraw current cell WITHOUT cursor (shows the char)
    xor a
    ld [wCursorOn], a
    call render_cursor_tile
    ; advance
    ld a, [wCurCol]
    inc a
    cp TCOLS
    jr c, .adv
    xor a
    ld [wCurCol], a
    call cursor_down
    jr .show
.adv
    ld [wCurCol], a
.show
    ld a, 1
    ld [wCursorOn], a
    call render_cursor_tile
    ret
.newline
    xor a
    ld [wCursorOn], a
    call render_cursor_tile
    xor a
    ld [wCurCol], a
    call cursor_down
    ld a, 1
    ld [wCursorOn], a
    call render_cursor_tile
    ret
.cret
    xor a
    ld [wCursorOn], a
    call render_cursor_tile
    xor a
    ld [wCurCol], a
    ld a, 1
    ld [wCursorOn], a
    call render_cursor_tile
    ret
.bs
    ld a, [wCurCol]
    or a
    ret z
    xor a
    ld [wCursorOn], a
    call render_cursor_tile     ; clear cursor at old
    ld a, [wCurCol]
    dec a
    ld [wCurCol], a
    ; blank the cell we backed into
    ld a, [wCurRow]
    call slot_addr
    ld a, [wCurCol]
    add l
    ld l, a
    ld a, h
    adc 0
    ld h, a
    ld a, $20
    ld [hl], a
    ld a, 1
    ld [wCursorOn], a
    call render_cursor_tile
    ret

; -----------------------------------------------------------------------------
; term_puts - HL = NUL-terminated string: print via term_putc.
; -----------------------------------------------------------------------------
term_puts::
    ld a, [hl+]
    or a
    ret z
    push hl
    call term_putc
    pop hl
    jr term_puts

; -----------------------------------------------------------------------------
; term_demo - drive the terminal: print enough lines to scroll, end at a prompt.
; -----------------------------------------------------------------------------
term_demo::
    ld hl, .msg
    call term_puts
    ret
.msg:
    db "GBOS terminal: cursor + fast scroll", 10
    db "01 the quick brown fox jumps", 10
    db "02 over the lazy dog.", 10
    db "03 line three of the scroll test", 10
    db "04 line four", 10
    db "05 line five", 10
    db "06 line six", 10
    db "07 line seven", 10
    db "08 line eight", 10
    db "09 line nine", 10
    db "10 line ten", 10
    db "11 line eleven", 10
    db "12 line twelve", 10
    db "13 line thirteen", 10
    db "14 line fourteen", 10
    db "15 line fifteen", 10
    db "16 line sixteen", 10
    db "17 line seventeen", 10
    db "18 line eighteen", 10
    db "19 line nineteen", 10
    db "20 line twenty", 10
    db "21 line twenty-one", 10
    db "22 line twenty-two", 10
    db "ready$ ", 0