Spectrum 48K

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The ZX Spectrum is a computer released in 1982.



CPU Z80 @ 3.5mhz
Ram 48K (16k contended, 32k non-contended)
Rom 48K
Display 256x192 pixels. Colour attributes 32x24 (16 available colours)
Sound beeper, bit bashed by software
keyboard 40 key rubber keyboard
Power 9v @ 1.4A unregulated, center -ve.


Memory map

 0000 (00000) -+---------------------+
               |    ROM area (16K)   |
 4000 (16384) -+---------------------+
               | Contended Ram (16K) |
 8000 (32765) -+---------------------+
               |Uncontended Ram (32K)|
 FFFF (65535) -+---------------------+


The lower 16k and upper 32K of memory are uncontended.

Accessing the memory from $4000-$7FFF is contended with the ULA used to generate the TV image.

Rather than using the normal z80 DMA processes, the ULA simply stops the clock going to the CPU when it needs access to Ram.

The base Spectrum provides one IO port at $FE, although a trick is done when reading the keyboard to read more values. Note that only A0 is decoded, so it also reponds to $FC $FA.. ect.

This port is used for the following:

Pin Read Write
D0 Keyboard in 1 Border colour.
D1 Keyboard in 2
D2 Keyboard in 3
D3 Keyboard in 4 Mic out
D4 Keyboard in 5 Speaker out
D5 Ear in unused




Pin Desc
1 Signal ground
2 UHF video signal

Power connector

Note the reverse polarity


Pin Desc
1 0v.
2 +9v 1.3A


Mono jack 3.5mm.png

Pin Desc
1 Gnd.
2 Audio in


Mono jack 3.5mm.png

Pin Desc
1 Gnd.
2 Audio out

Expansion connector


Pin Top Bottom
1 A15 A14
2 A13 A12
3 D7 +5V
4 n/c +9V
5 D0 0v
6 D1 0v
7 D2 Clk
8 D6 A0
9 D5 A1
10 D3 A2
11 D4 A3
12 /Int /IORQGE
13 /NMI 0V
14 /Halt Video [1]
15 /MReq Y [1]
16 /IORQ U [1]
17 /RD V [1]
18 /WR /BusRQ
19 -5V /Reset
20 /Wait A7
21 +12V A6
22 -12V A5
23 /M1 A4
24 /Rfsh /RomCS
25 A8 /BusAk
26 A10 A9
27 /NC A11
  1. 1.0 1.1 1.2 1.3 Video signals are handled differently on 128K and later model Spectrums, and these signals are N/C


See also..

How the DC-DC converter works

Spectrum introductory manual

Basic programming manual

Service manual