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 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
0xF000	4k	BDOS and hw drivers
	var	BDOS data
SP	var	Stack, growing down
		Free memory
0x0100	up to stack	Transient Program Area
0x0080	128 bytes	Command line and disk buffer
0x0000	128 bytes	System data area

Application program interface

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

File I/O is done via FCB blocks.

Zero page

Offset	Size	Usage
0x00	3 bytes	warm reboot jump instruction
0x03	byte	unused (would be iobyte)
0x04	byte	default drive
0x05	3 bytes	syscall jump instruction
0x08	8 bytes	8080 RST 1 interrupt code
0x10	8 bytes	8080 RST 2 interrupt code
0x18	8 bytes	8080 RST 3 interrupt code
0x20	8 bytes	8080 RST 4 interrupt code
0x28	8 bytes	8080 RST 5 interrupt code
0x30	8 bytes	8080 RST 6 interrupt code
0x38	8 bytes	8080 RST 7 interrupt code / debugger
0x40	varies	area reserved for usage by RDOS
0x80	byte	length of command line
0x81	127 bytes	command line with $length bytes plus a trailing 0x0D