(**160K**) in all. The +3 that followed it a few years later increased that to almost **192K** with an additional **32K** of ROM while the Next has increased that number even further to **1024K** or **2048K** depending on if you have expanded the ram on your machine or not. All the extra memory is hidden from the processor by the hardware using a process called paging – *NextBASIC* (and the processor) always *sees* the memory as **16K** of ROM and **48K** of RAM (or **64K** of RAM with no ROM in **allRAM** mode – though that is never used by *NextBASIC* and *NextZXOS* and it's reserved for CP/M). While the processor can indeed address only **64K** of memory at once, the extra memory can be slotted in and out of that **64K** at will as seen in the introduction to this chapter. Consider an old jukebox. Although it (and you) can only deal with one album at a time, there are many more albums there which can be selected with the right buttons. So, even though there's much more information than you can use at any one time, you can pick and choose which part is relevant. It is much the same for the processor. By setting the right bits in an I/O port, it can pick and choose which chunks of the available of memory it wants to use. When in non–banked usage of *NextBASIC* as well as when using legacy software most of the memory is ignored, but for Next mode games playing, *Layer 2* graphics and the use of all the new capabilities the ZX Spectrum Next is equipped with, having sixteen or even thirty two times as much RAM is really rather useful! Normally, usage of the additional memory capabilities are handled directly by *NextZXOS* and *NextBASIC* either automatically or by using the **BANK** commands, however in order to understand the underlying mechanisms we can elaborate a little bit. Look again at the memory map; RAM pages **2** and **5** are always in the positions shown when *NextBASIC* is used, though there's no reason why they shouldn't be in the "legacy banked" section (**C000h** to **FFFFh**) – however, it would be difficult to see any use for this. For legacy usage (usually where programs generate very strictly timed video effects), RAM banks are considered as being of one of two types: contended (meaning that there's a competition between the CPU and the ULA for access to them) and uncontended (meaning the CPU has their exclusive use). Only four banks are ever contended: banks **4** to **7**. The rest of the available RAM banks are always uncontended. This is a setting that can be turned on an off by using a Next Register as we saw in the previous chapter. It's turned OFF by default, but for compatibility reasons, *NextZXOS* turns it ON when loading software in a legacy format (**.SNA**, **.Z80** or **.TAP**). When writing software that may be used in older models, place any machine code which has critical timing loops (such as music) in uncontended banks[^p266-5]. Assuming contention has been turned ON, to turn it OFF you will need to issue a: ``` REG 8, % REG 8|@01000000 ``` command, setting therefore **NextREG 8, D6** to **1**. The inverse (setting it to **0**) will turn contention ON again for these banks. Alternatively you can just press the **NMI** button and set it/reset it using the *NMI menu* under *Settings > General* which is much much easier! The ZX Spectrum Next uses a combination of paging techniques we called *standard* at the beginning of this chapter. In reality, it uses three: The 128K style paging (described below) controlled by I/O address **7FFDh**, the +3 style paging controlled by I/O address **1FFDh** extended by Next Memory Bank Select control controlled by I/O address **DFFDh**. The reason for this complicated scheme is that the original ZX Spectrum 128K which introduced banking, only had 8 pages of RAM (**8** × **16K**) to deal with and only two of ROM (**2** × **16K**) so there was no appropriate care taken for further expansion. In an original 128K ma- [^p266-5]: For comparison, executing **NOP**s in contended RAM will give an effective clock frequency of approximately 2.6MHz as opposed to the normal 3.5MHz in uncontended RAM for the base clock speed. This is a speed reduction of about 25%