Upgrade described in Informik III/1987, in article
"Jak z ZX SPECTRUM zrobić komputer. Zmieniamy architekturę
komputera." ("How to make a computer from ZX SPECTRUM. We change
architecture of computer.") by Wojciech Apel.
The upgrade is very interesting. Especially because it was
designed in year 1985, the same time when ZX Spectrum 128K.
A latch was added as a port at 62 (#3E) address (ULA is disabled for A7=0 and A6=0). Ouputs of this latch are called S0, S1, S2, S3, S4 (in tables shown also as binary values range %00000..%11111). Also there are two extra switches to select confuguration. First of them is called TRYB and selects between ZX and CP/M modes. Second switch is called BANK and selects 32kB bank. As a result there are several possible configurations...
Memory map after upgrade
Table 1. Switch TRYB set to ZX.
S4=1 & BANK=1
S4=0 | BANK=0
S4=1 & BANK=1
S4=0 | BANK=0
Table 2. Switch TRYB set to CP/M.
S0=1 & S1=0
S0=1 & S1=1 & S3=0
S0=1 & S1=1 & S3=1
URAM0, URAM1, URAM2, URAM3 - 4 × 16kB "upper" RAM banks
VRAM - 16kB "lower" RAM with screen
ROM - original ROM
EPROM - 4kB or 8kB boot EPROM
URAM0* - with write protection
Please have a look for a link to alternative ROMs balow and the 80-LEC.ROM there. It has support for 32kB bank in lower half of memory, switched by bit 0 of #FD port. Data can be copied with MOVE command and strange syntax.
It is incredible, how easy could the ZX Spectrum 48K be upgraded to get 64kB RAM all RAM mode! (The ROM and video RAM are in
separate bank.) It was enough to replace 32kB RAM with 64kB chips and connect A15 to multiplexer input (in place of jumper). But
A15 line must be separated with 680ohm resistor - the ULA A15
input and the top memory logic circuit must be behind it.
Then we add 74LS74, of which we need only one latch is needed. The /R input should be connected to /RESET line, D input to one of lines D5..D7 and /CLK to /WR ORed /IORQGE ORed (with 680ohm resistor and diode). So the latch (1bit register) expands the 254 port used by ULA. When 0 is written, the normal 16kB ROM + 48kB RAM architecture is used. But when 1 is written to latch, its non-inverted output forces (with diode) high state on the part of A15 line sepated by resistor. So there's no access to ROM and VRAM, while "upper" memoy decoder activates RAM for all 64kB of address space.
Needed parts: eigth 4164 chips, one 74LS74, two 680ohm resistors, two 1N4148 diodes and some wires. Perhaps a switch to lock this feature for better compatibility with some strange written software, that uses
IN A,(#FE): OUT (#FE),A...
Links to other pages
- Alternative ROMs for ZX Spectrum
- Alternative ROMs for ZX Spectrum.
- 4MB RAM upgrade : ZX Spectrum
- ZX Spectrum memory upgrade - 4MB RAM (DRAM).