Add file I/O to the ABI

[thejpster]

Need to be able to read/write files and list directories from the Apps, e.g. loading bitmaps/code from disk.

[thejpster]

MS-DOS offered a bunch of virtual files you could open:

• COM1, COM2, ...
• AUX
• LPT1, LPT2...
• NUL
• etc

Linux/UNIX has virtual devices in /dev which acheive the same effect.

In Monotron the devices we want to perform file I/O (read, write, etc) on are:

• SDMMC interface (raw disk access)
• Files in a directory
• Directories (a special case of read) on a given volume
• Open volumes on a given disk
• Serial Interface (UART0 over USB and UART1 over TTL/RS-232) at various baud rates
• Raw SPI interface (with separate chip-select to the SD card)
• I2C interface (with addressing?)
• The text console (with cursor moves, and colours)
• The graphics frame buffer
• The keyboard (as CP850 bytes, as Unicode UTF8 bytes and as raw scan-codes for games).
• The joystick
• The synthesiser output (3 channels)
• The raw PCM output (into a ring buffer)

[thejpster]

An alternative approach is to provide a BIOS-level API (enumerating fixed disks, read/write sectors, etc) and moving file read/write into a DOS. This could be loaded into RAM or stored in ROM, but it would be distinct from the BIOS. CP/M does something similar. It would allow us to keep the OS the same on multiple platforms, while the BIOS deals with high-speed SDMMC vs 1-bit SPI, for example. It would effectively be a HAL.