====== System Interface ======

Emulated hardware is managed by writing to memory-mapped ports beginning at address $ff80.

  const SIbaseAddress=$ff80
  const SIrefreshOff=0, SIrefreshOn=1, SIgraphicsMode=2, SIkeyInterface=3;

  SIrefreshOff       Write <any> to force OFF register refresh and display animation
  SIrefreshOn        Write <any> to re-enable register refresh and display animation
  SIgraphicsMode     Write <number of colours> to select graphics mode (2, 4, 16 allowed values)
  SIkeyInterface     Write 0 then read an ASCII code (if<128), write 255 to clear the keyboard buffer
  
==== Refresh ====

This is a simple way of switching on or off the "Refresh" checkbox from within a running program. For example, it might be a good idea not to have animation during an intensive routine such as clearing the screen. Simply write any value to SIbaseAddress+SIrefreshOff to turn Refresh off, or to SIbaseAddress+SIrefreshOn to turn it on again.

  init:
    clr SIbaseAddress+SIrefreshOff
    jsr ClrScr
    clr SIbaseAddress+SIrefreshOn   

==== Graphics Mode ====

There are three bitmap graphics modes, differing by the resolution and colours available.

  2 colours, 1 bit per pixel
  4 colours, 2 bits per pixel
  16 colours, 4 bits per pixel
  
To select a mode, simply write the number of colours to SIbaseAddress+SIgraphicsMode

  Select16ColMode:
    lda #16
    sta SIbaseAddress+SIgraphicsMode

The emulator's core OS provides an alternative method via SWI3 with the service number passed in register A and the argumnet in B:

  Select4ColMode:
    lda #$02       ; A=service number 2 (graphics mode)
    ldb #$04       ; B=argument (4 colour mode)
    swi3

Now all writes to the graphics display area will be interpreted in this mode. Note that to save time this does NOT effect anything already on the display until the memory is read or written. So you may need to clear the screen after selecting the mode.

=== Graphics Screen Layout ===

In all modes the graphics RAM beings at $0600 and is 6K bytes long. In 2 colour mode it is 256 pixels wide by 192 pixels high. It is conventional to map the display with the top-left at (0,0) and the bottom right at (255,191) using the same virtual coordinates whatever the colour mode. Therefore in 4-colour mode pixels are 2*1 units in size, and in 16-colour mode 2*2 units.

  const grxBase=$0600, grxSize=$1800, gLineBytes2=$20, gLineBytes4=$20, gLineBytes16=$40

Using 2-colour mode as an example, memory layout is from screen left to right, then top to bottom. So pixels (0,0) to (7,0) are stored in the byte at offset 0. The most significant bit in the byte represents the leftmost pixel. To represent a row of 256 pixels takes 32 bytes. So we add 32 bytes to the base address for each row moved down, thus pixel (0,1) is stored in the byte at offset 32.

The 4-colour mode needs 2 bits for each pixel, thus to store pixels of colours of successively 0, 1, 2, 3 at coordinates (0,0), (2,0), (4,0), (6,0) would mean storing the binary value 00011011 at byte offset 0.

The 16-colour mode needs 4 bits per pixel, with 128 pixels (hence 64 bytes) per line, with only 96 lines. Thus to store colours 4 and 13 at coordinates (128,160) and (130,160) would mean storing the binary value 01001101 at byte offset $1420.

=== Palette ===

The colour palette used is the 16-colour [[https://pico-8.fandom.com/wiki/Palette|PICO-8 standard]] which provides a pleasant selection for retro video games. In 4-colour mode colours (0, 1, 2, 3) map to (0=black, 8=red, 11-green, 12=blue) respectively. In 2-colour mode colours (0, 1) map to (0, 11).

==== Keyboard ====

The keyboard is accessed by a single port. Simply write 0 to the port then read from the same address. A negative value means no key has been pressed, otherwise the value is the ASCII code of the last key pressed. Write $ff to the same port to clear the keyboard buffer.

  ; Read an ASCII character from the keyboard
  readChr:
    clra
    sta SIBaseAddress+SIKeyInterface
    lda SIBaseAddress+SIKeyInterface
    bmi readChr                        ; wait until bit 7 is zero
    cmpa #escape                       ; flag Zero if ESC
    rts