Two of the most important ports in this list are collectively called *Next Register* or *NextREG* for short. Most of the machine's features can be controlled through *NextREG*. ### Accessing the ZX Spectrum Next features with NextREG We use a *NextREG* by first selecting it with the control port and then writing to – or reading from – the *NextREG* data port. In *NextBASIC* this is achieved with two consecutive **OUT** commands in the case of writing or with a combination of consecutive **OUT** and **IN** in the case of reading from a NextREG. The first command is directed to the *Select* port **9275** (**243Bh**), selecting a specific register and the second to the *Data* port **9531** (**253Bh**) to modify or read the value stored there. If given from *NextBASIC*, these commands must be given consecutively in one line as *NextBASIC* may do something different with *NextREG* in-between commands. If you give the first and then wait to give the second, *NextBASIC* may have changed the *Select register* in the meantime; so by giving them together you give it no time to do something else. The Z80N CPU which powers the ZX Spectrum Next, also provides a special **NEXTREG** instruction and this is referenced in *Appendix A*. Finally, as mentioned in the introduction, in order to read *NextREG*, *NextBASIC* also has a specialised command and function called **REG**, which is much easier to use than the combination of **OUT** and **IN** keywords. We'll showcase both methods here, in order for you to be able to use either as there are cases where the ZX Spectrum Next's facilities are still available but without *NextBASIC* to provide access to them. The command as a statement has the form: **REG** *n,v* which is essentially the same as doing: OUT 9275, n:OUT 9531, v Obviously *n* is the register number and *v* is the value we modify the register with. As a function, **REG** has the following form: **% REG** *n* or **REG** *n* as it works with both the integer as well as the standard expression evaluators. This essentially is the same as executing **OUT 9275, n: %x** = **% IN 9531** (or the equivalent **x** = **IN 9531**). Let's give one simple example in both forms and let's mix-and-match a bit as well, to show the equivalency: Assuming we want to change speeds to **28MHz**, we could give: ``` RUN AT 3 ``` or ``` OUT 9275, 7:OUT 9531, 3 ``` and we can verify that it is set by either bringing up any *NextBASIC* menu (menus list the currently set speed on top) with the **EDIT** key or by doing: ``` OUT 9275,7: PRINT % IN 9531 & @11 ``` Which is the same as ``` PRINT % REG 7&@11 ```