## Components

The BIOS is in the uppermost 2K of the memory area - it provides basic character and disk I/O functions.
It it expected to stay static and not be unloaded by the user program.
The BIOS may hook into interrupts if necessary to provide its functionality.
Depending on your hardware, a different BIOS binary may be used.

The BDOS is in the 2K below the BIOS, it provides a filesystem driver to work atop of the BIOS.
It is agnostic to the underlaying hardware configuration, but different depending on the instruction set.
A user program might overwrite memory up to the end of the BDOS.
Upon program exit, the BIOS will reload the BDOS from disk.

## Memory Layout

On a 8080, there are no segments, the addresses are as displayed.

On on the 8086, a single segment with a value of 0x0100 is assumed.
This implies that the first 4k of memory are not used.

The BIOS and BDOS may be recompiled for starting at a lower address, in this case,
the minimum memory requirement may be less than 64k.

|Start |Size     |Function|
|------|---------|--------|
|0xF800|2k       |BIOS / hw drivers|
|0xF000|2k       |BDOS|
|...   |...      |BDOS data area, disk deblocking buffers|
|SP    |var      |Stack, growing down|
|...   |...      |Free memory|
|...   |var      |Heap, growing up|
|0x0100|var      |Transient Program Area|
|0x0080|128 bytes|Command line and disk buffer|
|0x0008|120 bytes|BIOS data area|
|0x0005|3 bytes  |Jump code for BDOS|
|0x0004|1 bytes  |default drive for BDOS|
|0x0003|1 bytes  |may be used as iobyte by BIOS|
|0x0000|3 bytes  |Jump to BIOS warm boot entry point / program exit|

## Application program interface

It is using the `call 5` convention with CP/M compatible syscall numbers.

File I/O is done via FCB blocks.