Chapter 23 – The Memory

Overview

In previous chapters, we talked about binary code, bytes, words and long words. We also discussed strings, floating point and integer numbers. It's time to go into more detail and explore how your computer stores information we put into it.

The kind of data we're processing makes absolutely no difference to the computer. Whether it's music, a game or a document, it ends up as a series of ones and zeros organised as bytes and stored in memory. We can rely on NextBASIC to manage that information or we can do it ourselves as long as we know how!

The ZX Spectrum Next is an 8-bit computer with a 16-bit Address Bus. That means that it stores and manipulates information in 8-bit bytes, and can see at most 65536 of these bytes at one time. Hold onto this information for now as it's important.

ROM and RAM

Memory can be categorized into two kinds: ROM and RAM. ROM (read-only memory) cannot be written to whereas RAM (random access memory) can be both read and written. RAM is where things like the program and display contents are stored because they can change while the computer is running. ROM can be used to hold something permanent like the NextBasic interpreter or NextZXOS. You may have picked up on the discussion of the ROM earlier and may have been wondering how we can load a ROM from a file as described in various places around this book, when ROM is supposed to be permanent and read-only (see for example Chapter 1).

The truth of the matter is that, although the ZX Spectrum Next contains a physical ROM chip, this has nothing to do with the ZX Spectrum Next's operation. The physical ROM is used to configure the Xilinx FPGA and a small amount is used to store a program that configures the machine on boot. The ZX Spectrum Next itself only sees RAM memory supplied by up to four 512K SRAM chips. The unexpanded model has two chips present for a total of 1024K of memory and the expanded model has four for a total of 2048K memory (See Chapter 22 on how to upgrade the RAM to the maximum possible). The ROM contents are loaded into a portion of this RAM and then the hardware is instructed to make that portion read-only. So after the machine boots, those areas of RAM behave just like ROM because running programs cannot change anything stored there. This reproduces the behaviour of the original Spectrums which did use physical ROMs to store the basic interpreter. In other words, for the purposes of NextBASIC and NextZXOS the ZX Spectrum Next indeed has ROM.

The Memory Map

In the introduction of this chapter we talked about how the ZX Spectrum Next has a 16-bit Address Bus and how this fact means the computer can see 65536 bytes (64 Kilobytes) of memory, a figure that includes both ROM and RAM. That is enough to generate the obvious question: But my computer has 16 (or 32) times as much memory, what's the point of having it? And you would be absolutely right to ask this!

The answer to that question is that the computer uses a memory access technique known as bank switching. In this technique there's a distinction between the maximum addressable memory (the amount of memory that the CPU can see, ie 64K in our case) and the amount of physical memory in the system. In the ZX Spectrum Next's case, the physical memory is divided into equally sized portions called banks and the 64K of memory that the computer can see is also divided into the same sized portions called slots. A virtual map of sorts is constructed that tells the hardware what physical memory bank appears in each of the 64K's slots. We shall refer to this virtual map as the memory map. Whenever information located in physical memory is required, the specific physical bank that holds it is entered into the memory map in one of its slots so that the CPU can see the bank in the slot's address range. Paging in the new bank replaces whatever was there before be-