The lower 16K is the Spectrum ROM (Read-Only Memory) and holds the machine code instructions used to implement ZX BASIC and handle the hardware, such as the keyboard, speaker.

A 2718 EPROM containing your own Spectrum ROM can replace the ROM, if you wish (beyond the scope of this guide). The upper half is the Spectrum RAM (Random Access Memory) which is used primarily to store user data. The first 16K of which is used to hold the screen display (6.75k), various system variables (discussed later), and user programs/data. This memory represents the 16K Spectrum version. The 48K spectrum version has an addition 32K of RAM on top of the 16K which is used to hold extra user program/data.

The 16K RAM uses 8 standard 4116 16K bit chips and the 32K RAM expansion uses eight 4532 32K bit chips. The 4532 chips are/were produced by Texas Instruments and are actually 'failed' 64K chips.

To upgrade/replace a Spectrum RAM you need to know which version you have. ISSUE 1 boards can be determined by removing the lower half of the case and observing anything written to the right of the Ear/Mic sockets. ISSUE 2 boards are marked on the front edge of the circuit board, just right of centre. ISSUE 2 machines can be upgraded by soldering new chips on the board. ISSUE 1 boards cannot be upgraded as easily because a new circuit board is required.