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'
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.