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
0x006C	20 bytes	Pre-filled FCB 2
0x005C	16 bytes	Pre-filled FCB 1
0x0008	84 bytes	BIOS data area, specific to BIOS implementation
0x0005	3 bytes	Jump code for BDOS
0x0004	1 byte	default drive for BDOS
0x0003	1 byte	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.