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