| 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)