[sebhc] Schematic

[Lee Hart]

Carroll Waddell wrote:
> How do you get an 8080 to address more than 64K?

Basically, the same way Intel got the 8088 to address 1 megabyte. You
add segment registers, which supply the upper bits of the address, and
let the CPU itself supply the lower bits

I designed a product called the "Databug" which is a Z80 single-board
computer, 6.25" x 2.5", 1 bytewide RAM socket (8k-512k bytes), 1 EPROM
socket (8k-512k bytes), 2 serial ports, 3-channel A/D converter, and an
8-bit parallel I/O port. It's used for data logging and processing
applications.

The two memory chips can total 1 megabyte, which requires 20 address
lines (A0-A19). The Z80 only has 16 address lines (A0-A15). So, 4 bits
of an 8-bit output port provide the upper 4 address lines (A16-A19). Let
N = the value of these 4 bits; 0 upon Reset, and whatever the Z80 writes
thereafter.

A 5th bit of this port also provides a RAM/ROM select bit (high=RAM,
low=ROM). A 74HC139 decodes the Z80's A15, and the output port's RAM/ROM
bit as follows:

    	   A15=0	A15=1
RAM/ROM=0  ROM bank N	RAM bank 0
RAM/ROM=1  RAM bank N	RAM bank 0

On power-up or Reset, RAM/ROM=0 and N=0 (A16-A19 all 0). So 0-32k is the
lowest 32k in the EPROM, and 32k-64k is the lowest 32k in the RAM.

If the Z80 writes some number N to the output port, then 0-32k will now
address the selected 1 of 16 32k banks in the ROM. If the Z80 sets
RAM/ROM=1, then 0-32k now addresses RAM bank N (1 of 16 32k blocks in
the RAM chip). Since the upper half of the Z80's address space is always
RAM bank 0, you have a "safe" place to put the code that manages the
bank switching.

This system was used just because it was simple (just a 74HC139 and a
74HC02 to implement it). But we could work out variations on it for more
memory chips, or for different size memory blocks.

-- 
"Never doubt that a small group of committed people can change the
world. Indeed, it's the only thing that ever has!" -- Margaret Meade
--
Lee A. Hart  814 8th Ave N  Sartell MN 56377  leeahart_at_earthlink.net


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