This is a unique book which I'm sure many users of the BBC Micro will find invaluable as it covers, almost completely, how the Sideways ROM system works. It will appear both to those who like to know how their micro works and to those who wish to write their own Sideways ROM software.

Writing your own software is not that difficult and now that EPROM programmers are becoming cheaper (or through theuse of sideways RAM) I'm sure many will want to have a go. For the practical types a simple circuit, plus software, for an EPROM programmer is given in the book.

The book is in four main parts. First, the Operating System routines which control the sideways ROMs are discussed; all the service calls (including those for the ROM filing systems) are described.

Unfortunately there is one error - Acorn DFS does not allocate &E00 to &1900 as absolute workspace; &1700 to &1900 is its private workspace. One omission is how utility ROMs can generate error messages which can be read by language ROMs. The usual system is to put them at the very bottom of the stack (&100 upwards).

The book goes on to a very useful and detailed examination of two commercial ROMs and one developed within the book. This should give ample information and ideas on writing your own ROM software.

The third section has a look at the range of commercial utility ROMs which are available, especially Basic toolkits and machine code monitors. It's unfortunate that the picture of Beebug's new Help ROM shows a mistake in the ROM.

Finally there is a section containing bar code listings of all the major programs within the book - very useful, as I'm sure this form of software distribution will quickly catch on. For those who prefer, the programs are also available on disc.

In summary, then, apart from a couple of minor omissions, a most useful book.