2021-01-19 08:36:01 +01:00
|
|
|
; *******************************************
|
|
|
|
|
; * PAT80 COMPOSITE PAL VIDEO ADAPTER *
|
|
|
|
|
; * Character generator module *
|
|
|
|
|
; *******************************************
|
|
|
|
|
|
|
|
|
|
; This module generates the character pixels using the font present in rom
|
2021-01-24 11:27:40 +01:00
|
|
|
; and adds it on the framebuffer in the position indicated by POS_COARSE (Y).
|
2021-01-22 08:58:11 +01:00
|
|
|
|
2021-01-24 11:27:40 +01:00
|
|
|
.equ LINE_COLUMNS = 52 ; number of columns (characters or chunks) per line
|
|
|
|
|
|
|
|
|
|
; Draws character in register A to the screen at current coords (Y)
|
|
|
|
|
; @param A (r0) ascii code to display
|
|
|
|
|
; @modifies r0 (A), r1, r2, r3, r16 (HIGH_ACCUM), Y, Z
|
|
|
|
|
draw_char:
|
|
|
|
|
; Glyph's first byte is at:
|
|
|
|
|
; glyph_pointer = font_starting_mem_pos + (ascii_code * number_of_bytes_per_font)
|
|
|
|
|
; But all the fonts are 1 byte large, so a glyph is 1*height bytes:
|
|
|
|
|
; glyph_pointer = FONT + (ascii_code * FONT_HEIGHT)
|
|
|
|
|
|
|
|
|
|
; Save current chunk cursor position (Y)
|
|
|
|
|
mov r2, YL
|
|
|
|
|
mov r3, YH
|
|
|
|
|
|
|
|
|
|
; Load first glyph position on Z
|
|
|
|
|
ldi ZH, high(FONT)
|
|
|
|
|
ldi ZL, low(FONT)
|
|
|
|
|
; Obtain offset multiplying ascii_code * number_of_bytes_per_fontr1
|
|
|
|
|
ldi HIGH_ACCUM, FONT_HEIGHT
|
|
|
|
|
mul A, HIGH_ACCUM ; result overwrites r0 and r1!
|
|
|
|
|
; 16-bit addition between gliph's first byte position and offset (and store result in Z)
|
|
|
|
|
add ZL, A
|
|
|
|
|
adc ZH, r1
|
|
|
|
|
; Z contain our glyph's first byte position: draw it
|
|
|
|
|
; The drawing consist of FONT_HEIGHT cycles. Every glyph byte is placed on its own line
|
|
|
|
|
; on screen. To do this, we place it LINE_COLUMNS bytes after the previous one.
|
|
|
|
|
ldi HIGH_ACCUM, FONT_HEIGHT
|
|
|
|
|
draw_char_loop:
|
|
|
|
|
; Load glyph line byte from program memory (and point to the next)
|
|
|
|
|
lpm r1, Z+
|
|
|
|
|
; Write glyph line to framebuffer at chunk cursor position (Y)
|
|
|
|
|
st Y, r1
|
|
|
|
|
; Increment chunk cursor position (Y) to next line of the same char column
|
|
|
|
|
adiw YH:YL,LINE_COLUMNS
|
|
|
|
|
; Decrement loop counter and exit if reached 0
|
|
|
|
|
dec HIGH_ACCUM
|
|
|
|
|
brne draw_char_loop
|
|
|
|
|
; Char drawing is complete. Set chunk cursor position to next char first line
|
|
|
|
|
mov YL, r2 ; first restore Y
|
|
|
|
|
mov YH, r3
|
|
|
|
|
adiw YH:YL,1 ; just increment pre-char-drawing-saved chunk cursor position by 1
|
|
|
|
|
ret
|
