MTX Plus⁺ CP/M BIOS

All CP/M systems have their own Customised Basic I/O System (CBIOS, or just BIOS), used to interface the machine specific hardware with the high level CP/M function calls for such items as the keyboard, video display and disk drives. The CP/M BIOS and BDOS are combined into a single module loaded into high memory sometimes referred to as FDOS (Floppy Disk Operating System).

The start of the BIOS code contains a table of sixteen 8080 jump instructions, each of the jump instructions will execute one of sixteen possible CP/M 2.2 BIOS functions. The first jump instructions is at memory location 0000 and points to the wboot  (Warm Boot) function, since 8080 jumps instructions are three bytes in length, memory locations 0001 and 0002 contain the start address of the BIOS jump table

Apart from the wboot function, all the other BIOS functions are one of two types - serial or disk functions. The serial oriented BIOS functions transfer one ASCII character at a time to or from hardware devices and/or provide simple status about the devices. Under CPM2.2 there are four logical serial devices; Console (CON), List (LST), Punch (PUN), and Reader (RDR). Each of the four logical serial devices can be mapped to any of four possible hardware devices.

Memory address three contains the logical serial device mappings this memory location is referred to as the IOBYTE. The IOBYTE uses two bits per logical device to select one of a possible four hardware devices. Under CPM2.2 a program should use the BDOS service functions seven and eight to read and write the IOBYTE.

The list of BIOS functions in CP/M 2.2 are shown in the table below. (Some BIOS functions are duplicated in the BDOS, the value in parenthesis is the corresponding BDOS function.)

With the functions listed, there just remains the small matter of creating the software for MTXPlus⁺ !!!