Ancient Computing

These are some sketches and notes for a couple of new obsolete computers.

Z80
The first is a single board Z80 system.

1 meg of RAM, 1 meg of EPROM, 2 serial ports, 4 timer/counter channels, and
24 bits of I/O. There are also 8 single bit output ports (via 75LS259) and 8 input
bits (4 used for initial baud rate).

The printed circuit board size is 4-1/2" tall by 6 inches wide - double sided.

Virtual memory mapping
The sketch below shows how the virtual memory addressing is performed.
Four output bits are latched and ORed with A15 to produce the virtual address.
The A15 gating insures that the upper 32k of memory will always be available.
this is where the code running the system should be located so that the driver code
doesn't disappear when the banks are changed. Intended to run CP/M.

XT
The next one is a single board 8088 system running at a blaszing 8 megahertz!

512K of RAM, 512K of EPROM, 1 serial port, 3 timer/counter channels, and
24 bits of I/O. There are also 8 single bit output port (via 75LS259) and 8 bit
level inputs.

This circuit board is 5" x 7-1/4" - double sided.
It uses the basic Intel 8088 XT chip set and will run MSDOS 6.2 (in the form of FreeDos).