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generator: pandoc
title: '8 Bit Laptop: Bank Switching Memory'
viewport: 'width=device-width, initial-scale=1.0, user-scalable=yes'
---
2018-04-12T14:40:53+10:00
I have been trying to wrap my head around exactly how the 74LS610 Memory
Management Chip will work in my Hackaday Prize competition entry. There
is not a lot of complete documentation on exactly how to implement the
74LS610. So, ultimately, I decided to scrap using the chip, and I
decided to use a simpler and more rough-and-ready solution to how to
extend the memory of this internet-enabled 8 bit laptop I am building.
I did some searching, and I found a solution I can actually understand:
<http://www.zcontrol.narod.ru/diagrams/ZramBankSwitch.pdf>.
This document outlines how to extend the ROM of a computer to 128K
bytes, and the RAM to 512K bytes. This is the schematic:
The incredible benefit of this solution is that it minimises the amount
of programming needed in order to switch banks of ROM and RAM.
Selecting Between the ROM and RAM
---------------------------------
The implementation uses one half of a 74HCT139 decoder to select between
the ROM and RAM. When A15 is high, RAM is selected; when A15 is low, ROM
is selected. A15 is gated with /MREQ to assure that the memory chips are
enabled only when the bus access is for memory, and only when the
address lines are stable.
The Bank Switch Latch
---------------------
As the document outlines, a 74HCT273 eight-bit latch serves as the bank
switch latch. Because this circuit is intended for a Z80 processor, the
circuit requires the user to output bank switch information with a Z80
OUT instruction, which would generate the output strobe /BANK. Because
we are using a 6502, I imagine we could do that with the VIA.
The document specifies that a 74HCT138 or similar part (not shown in
this schematic) would interface with the Z80 processor to recieve an OUT
opcode to generate the bank switch strobe and other needed I/O strobes.
At power-on, the /RESET signal resets all the bank switches to zero.
The Eight AND Gates
-------------------
The latched bank switch output signals, BS0 through BS7, are connected
to one input of each of eight 74HCT08 AND gates. The outputs of these
gates are used to generate extra address lines for the ROM and RAM
chips---three for the ROM, and five for the RAM.
Using A14 to Gate the Bank Switching
------------------------------------
Each of the eight 74HCT08 AND gates has its other input tied to Z80
address line A14. When A14 is high, the extra address lines will follow
the values latched in the 74HCT273 bank switch latch. When A14 is low,
the extra address lines will all be set low, regardless of the values
latched in the bank switch latch. However the last values stored in the
bank switch latch remain there, ready for the next access to a
bank-switched memory area.
Very Simple!
------------
As you can see, this is a very simple solution to what could be a
complex problem. Using the VIA chip already being used in this design,
and just by using A14 and A15 of the 6502, we are able to increase our
total ROM addressable to 128K bytes, and RAM to 512K bytes. I think this
will be sufficient.
The Memory Map
--------------
The document we are working with specifies this as the memory map of the
design:
There are four areas in the memory map. The power of this design is that
there are 16K bytes each of ROM and RAM which are base ROM and RAM. This
means that these areas are not switched out upon switching banks, which
means permament program information can be stored in these banks, which
is very useful. I imagine we can section of parts of these 16K byte
areas for IO. The stack can be located in the base RAM area, and the
reset vectors needed for the 6502 can be located in the base ROM area.
There will need to be an area in the base RAM area which shadows the
state of the bank switch latch. This is so the operating system of the
computer can keep track of the state of the bank switch latch. This will
be useful for multi-tasking applications.