KAPE GPU commands

CMD FIFO - implemented in hardware as an abstraction layer between the CPU bus and the KAPE GPU.

The CPU sends commands through the FIFO and sees it as a normal IO in address-space, and sending bytes to this address
is then read by the GPU from the FIFO and ingested here. After a reset event - either hardware or software - the GPU needs around 100 ms
before it can start accepting commands. This is about 6-7 frames (at 61,7 fps).

The command list:
0x00        Send single character as textmode. 1 parameter. The next character is interpreted as a text character, and respects
            some special characters like enter and bs
 - 1 byte   The character to draw on cursor position. The color will be set as the current draw color bg/fg
0x01        Set draw color for text. 1 parameter. The next character is interpreted as the draw color.
 - 1 byte   0bFFFFBBBB
   - F      The 4-bit foreground color
   - B      The 4-bit background color
0x02        Enter Terminal mode. Terminal mode is 32x24 textmode. Future communications are interpreted as characters to be output
            to the char location. Char location is incremented. Upon entering, the foreground and background color are reset to
            Light Gray, and cursor position is set to 0,0. Cursor is shown and blinking. Screen is cleared to space. While in terminal
            mode, sending an ESC(27 or 0x1b) accepts a terminal command.
            See Terminal Commands for terminal commands.

0x10        Set single index to character or pattern table at coordinates. 3 parameters.
 - 1 byte   0b***XXXXX
   - X      The X position, from 0 - 31 (5 bits)
 - 1 byte   0b***YYYYY
   - Y      The Y position, from 0 - 23 (5 bits, clamped to 23)
 - 1 byte   Index to set at position X,Y

0x20        Set sprite data. 4 parameters
 - 1 byte   0bA*YYYXXX
   - A      Active (processed and active in drawing loops, not necessary drawn)
   - Y      3 bit value for the Hotspot Y (pixel considered as the origin, ie. offset for drawing algorithms)
   - X      3 bit value for the Hotspot X
 - 1 byte   X position
 - 1 byte   Y position (wrapped around from 192)
 - 1 byte   Tile index - uses the same tilemap as the background rendering
0x21        Set sprite active. 1 parameter
 - 1 byte   Sprite index to activate
0x22        Set sprite not active. 1 parameter
 - 1 byte   Sprite index to deactivate
0x23        Set sprite draw. 1 parameter
 - 1 byte   Sprite index to activate
0x24        Set sprite active. 1 parameter
 - 1 byte   Sprite index to activate
0x25        Set sprite tile index. 2 parameters
 - 1 byte   Sprite index to set the tile index of
 - 1 byte   Tile index to set to the sprite
0x26        Set sprite X. 2 parameters
 - 1 byte   Sprite Index
 - 1 byte   Sprite X coord
0x27        Set sprite Y. 2 parameters
 - 1 byte   Sprite Index
 - 1 byte   Sprite Y coord
0x30        Set sprite alpha color. 2 parameters
 - 1 byte   Sprite Index
 - 1 byte   0b****AAAA
   - A      The Alpha color that's not drawn
0x31        Set sprite X and Y coords. 3 parameters
 - 1 byte   Sprite Index
 - 1 byte   Sprite X coord
 - 1 byte   Sprite Y coord

0x40        Switch to text mode (draw from chargen table)
0x41        Switch to graphics mode (draw from pattern table, sprites supported)
0x42        Switch to combined mode (draw from both, use a bitmask to select which one to use)
0x43        Switch to lores mode (128x96 pixelbuffer, resides in pattern table, no sprites)

0x4A        Clear screen. 1 paramater. Fills either tilemap/text framebuffer or 128x96 bytes of pattern table, depending on the mode.
 - 1 byte   Byte to set to all memory.

0x4B        Flush frame to FB FIFO. No parameters.
            In Lores mode, the current framebuffer is sent to the FIFO only with the flush command.
            This is due to the fact that the CMDQ is only 2K in size, and thus to send a full frame to the FIFO you need a bit over 3 frames
            to do it. This way, you can use the internal buffer as a double-buffer, and only update the screen when done. Prevents screen tearing
            and provides possibility to update frames smoothly at ~25 FPS.

0x4C        Draw a pixel. 3 parameters
 - 1 byte   X position. Moduloed w/ SCR_LORES_WIDTH (128)
 - 1 byte   Y position. Moduloed w/ SCR_LORES_HEIGHT (96)
 - 1 byte   Color. Anded with a 0xf to prevent overflow.

0x4D        Draw a line. 5 paramaters
 - 1 byte   X1 position. Moduloed w/ SCR_LORES_WIDTH (128)
 - 1 byte   Y1 position. Moduloed w/ SCR_LORES_HEIGHT (96)
 - 1 byte   X2 position. Moduloed w/ SCR_LORES_WIDTH (128)
 - 1 byte   Y2 position. Moduloed w/ SCR_LORES_HEIGHT (96)
 - 1 byte   Color. Anded with a 0xf to prevent overflow.

0x50        Set combined mode bitmask for coordinate. 2 parameters.
 - 1 byte   0bB**XXXXX
   - B      The bit the bitmask will be set to. 1 draw from pattern table (graphics). 0 draw from chargen (text)
   - X      The X position, from 0 - 31 (5 bits)
 - 1 byte   0b***YYYYY
   - Y      The Y position, from 0 - 23 (5 bits but clamped to 23)

0x80        Send pattern table data for single element. 1 parameter + 32 bytes of data (4 bits per pixel)
 - 1 byte   The pattern table index to insert the data to
 - 32 bytes The pattern table data arranged as a sequential array of packed pixels, where one byte holds two pixels.

0xfe        Direct Framebuffer mode. Intended for Lores mode for piping data directly to the framebuffer. NB: GPU soft reset
            can't be activated in this mode! Reset signal needs to be manually sent to GPU through the ~RESET line.

0xff        Reset GPU. Once received, uses watchdog to simulate a hardware reset. Resets device.
            In case the last command has not finished, to properly FORCE a reset from a stuck state, we need to send
            as many + 1 reset commands as the longest fifo command queue, which is the send pattern table data at 33 bytes.
            So to do a FORCE RESET, send 34 reset commands to the CMD Fifo


Terminal commands. In Terminal mode, send ESC and command with parameters. Terminal mode accepts some control characters, like

0x0A LF     Line Feed. Goes row down, scrolls if too low.
0x0D CR     Carriage Return. Cursor moved horizontally to Column 0
0x1B ESC    Escape. The next character describes a command. See the following list.
0x08 BS     Backspace. Delete character on the left (wrap around to higher row) and replace with space

0x01        Set draw color for text. 1 parameter. Binary version. High nibble is foreground color, low nibble is background color
 - 1 byte   0bFFFFBBBB
  - F       Foreground color nibble
  - B       Background color nibble

0x63 'c'    Set draw color for text. 2 parameter. The next 2 chars '0-9'|'A-F' are interpreted as foreground and background colors
 - 1 byte   Foreground color '0-9'|'A-F'
 - 1 byte   Background color '0-9'|'A-F'

0x02        Set draw decoration. 1 parameter. Next character decides Bold, Italics, Underline font, Blinking
 - 1 byte   0b000SEUIB
   - B      Bold
   - I      Italics
   - U      Underline
   - E      Effect (Blinking)
   - S      Strike-through

0x03        Set Cursor Position. 2 parameters.
 - 1 byte   Cursor Column. Mod 32
 - 1 byte   Cursor Row. Mod 24

0x04        Set Cursor Column. 1 parameter
 - 1 byte   Cursor Column. Mod 32

0x05        Set Cursor Row. 1 paramater
 - 1 byte   Cursor Row. Mod 24

0xff        Exit out of Terminal Mode. The next character sent is interpreted as a normal GPU command.