Science of Cambridge MK14 Memory Map
Only the first 12 bits of the program counter are decoded

000-1FF 512 byte SCIOS ROM Decoded by 0xxx
200-3FF ROM Shadow / Expansion RAM
400-5FF ROM Shadow / Expansion RAM
600-7FF ROM Shadow / Expansion RAM
800-87F 128 bytes I/O chip RAM Decoded by 1xx0
880-8FF I/O Ports Decoded by 1xx0
900-9FF Keyboard & Display Decoded by 1x01
A00-AFF I/O Port & RAM Shadow
B00-BFF 256 bytes RAM (Extended) / VDU RAM Decoded by 1011
C00-CFF I/O Port & RAM Shadow
D00-DFF Keyboard & Display Shadow
E00-EFF I/O Port & RAM Shadow
F00-FFF 256 bytes RAM (Standard) / VDU RAM Decoded by 1111

Keyboard Decoding

On a READ the switch levels are read onto the bus via IC11 into bits D4-D7

On a WRITE the bits D0-D7 are latched (sort of !) via IC9 and IC10 into the video display latches (D0=Segment A,D7 = Decimal Point)

On any Access the lower 4 address bits are latched into IC12. These are decoded by IC13, dragging the appropriate line low to light the segments selected via IC9 and IC10

IC11 is normally read as 1111xxxx - if a key is pressed when the corresponding line is pulled low via IC13 then bits will be read as zero - this indicates a key is being pressed.

RAM Expansion

It is a standard Sinclair design that the 3 ROM Shadows between 0200 and 07FF can be replaced by RAM, giving a theoretical total of 1.5k (expansion) 1/4k (extended) 1/8k (RAM I/O Chip) 1/4k (Standard)

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