terminal/framebuffer.vhd

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VHDL
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library ieee;
use ieee.std_logic_1164.all;
use ieee.std_logic_unsigned.all;
use ieee.numeric_std.all;
use std.textio.all;
use ieee.std_logic_textio.all;
entity framebuffer is
generic (
input_clk: integer
);
port (
clk: in std_logic;
x: in std_logic_vector(9 downto 0);
y: in std_logic_vector(8 downto 0);
rgb: out std_logic_vector(23 downto 0);
cursor_row: in unsigned(5 downto 0);
cursor_col: in unsigned(6 downto 0);
-- write access to the character and row length rams
charbuf_we: in std_logic;
charbuf_wa: in std_logic_vector(12 downto 0);
charbuf_di: in std_logic_vector(7 downto 0);
rowlen_we: in std_logic;
rowlen_wa: in std_logic_vector(5 downto 0);
rowlen_di: in std_logic_vector(6 downto 0)
);
end framebuffer;
architecture logic of framebuffer is
type rom_type is array(0 to 127) of std_logic_vector(63 downto 0);
impure function read_font(filename: in string) return rom_type is
file rom_file: text is in filename;
variable rom_line: line;
variable rom: rom_type;
begin
-- skip comment in first line
readline(rom_file, rom_line);
for i in rom_type'range loop
readline(rom_file, rom_line);
hread(rom_line, rom(i));
end loop;
return rom;
end function;
constant font: rom_type := read_font("font.hex");
signal row: unsigned(5 downto 0);
signal col: unsigned(6 downto 0);
signal eff_row_wide: unsigned(6 downto 0);
signal eff_row: unsigned(5 downto 0);
signal read_address: std_logic_vector(12 downto 0);
signal current_char: std_logic_vector(7 downto 0);
signal current_row_length: std_logic_vector(6 downto 0);
signal current_glyph: std_logic_vector(63 downto 0);
signal current_glyph_valid: boolean;
-- delay by 2 cycles to match the delay of x/y -> rgb
constant glyph_pos_length: integer := 2;
type glyph_pos_type is array(1 to glyph_pos_length) of integer range 0 to 63;
signal glyph_pos: glyph_pos_type;
begin
row <= unsigned(y(8 downto 3));
col <= unsigned(x(9 downto 3));
-- calculate effective row with respect to the cursor; this keeps the
-- cursor always at the bottom of the screen and old contents scoll up
eff_row_wide <= resize(cursor_row, 7) + row + 1;
eff_row <= resize(eff_row_wide, 6) when eff_row_wide < "0111100" else -- 60
resize(eff_row_wide - "0111100", 6); -- 60
read_address <= std_logic_vector(col) & std_logic_vector(eff_row);
terminal_buffer: entity work.ram_2port generic map (
WIDTH_BITS => 8,
DEPTH_BITS => 13
) port map (
clk => clk,
ra => read_address,
do => current_char,
we => charbuf_we,
wa => charbuf_wa,
di => charbuf_di
);
-- store the length of every row so that we don't have to overwrite
-- existing rows when wrapping around and can simply disable display
-- of non-overwritten characters
row_lengths: entity work.ram_2port generic map (
WIDTH_BITS => 7,
DEPTH_BITS => 6
) port map (
clk => clk,
ra => std_logic_vector(eff_row),
do => current_row_length,
we => rowlen_we,
wa => rowlen_wa,
di => rowlen_di
);
-- as the row length is only saved after switching to the next row, we use
-- the current cursor col position in the case of the current row, which
-- is always at the bottom, so it is always row 59
current_glyph_valid <= col < cursor_col when row = "111011" else -- 59
col < unsigned(current_row_length);
process(clk)
variable current_glyph_pos: integer range 0 to 127;
begin
if rising_edge(clk) then
current_glyph_pos := to_integer(unsigned(current_char));
current_glyph <= font(current_glyph_pos);
end if;
end process;
delay_glyph_pos:
process(clk)
variable combined: std_logic_vector(5 downto 0);
begin
if rising_edge(clk) then
combined := y(2 downto 0) & x(2 downto 0);
glyph_pos(glyph_pos_length) <= to_integer(unsigned(combined));
-- move all elements one index down
for i in 1 to glyph_pos_length - 1 loop
glyph_pos(i) <= glyph_pos(i + 1);
end loop;
end if;
end process delay_glyph_pos;
-- actually currently BRG
with current_glyph(glyph_pos(1)) = '1' and current_glyph_valid
select rgb <=
"111111111111111111111111" when true,
"000000000000000000000000" when false;
end logic;