rent state of the port. The port at 1FFDh doesn't just control the ROM switch, so setting the variable to absolute values would be very unwise. Using AND/OR with a bit mask or SET/RES instructions is the preferred method of updating the variable.

Just as BANK678 reflects the last value output to 1FFDh, BANKM should also be kept up to date with the last value output to 7FFDh. Again, it is unwise to use absolute values, as the port is used for other purposes. For example, the bottom 3 bits of the port are used to select the RAM page that is switched into the memory area C000h through FFFFh (this is also shown in the above example). Naturally, when more than one bit is to be set/reset, a bit mask used with OR/AND is the more efficient method. Note that RAM paging was described in the Memory Management section in Chapter 24.

The above was a very simple example of calling DOS routines. It works – apart from the ZX Spectrum Next – on the ZX Spectrum +3 and ZX Spectrum +3e as well.

Opcodes Prefixes

Some Assembler opcodes are preceded by a prefix byte which changes the opcode represented by the following byte.

Assembler opcode prefixes CBh (203) and EDh (237) alter the meaning of certain instructions, as indicated in the 5th and 6th columns of Appendix A. This includes the provision of some entirely new opcodes for the ZX Spectrum Next.

Assembler opcode prefixes DDh (221) and FDh (253) alter the meaning of certain instructions that ordinarily refer to the H or L registers, so that they refer to either the component registers of IX or IY register respectively. For example, the instruction LD H,n will load the value of n into the H register. Preceding this two-byte instruction with the IX register's opcode prefix DDh, would result in the most significant 8 bits of the IX register being loaded with that value instead.

This general transformation rule is modified when the original instruction contains (HL), with this component replaced by (IX +N) and any other reference to HL left unaffected. For instance:

DDh 66h is interpreted as ld h,(ix + N)

A DDh opcode will be ignored, interpreted as nop, if it precedes DDh, EDh or FDh. Similar rules apply to the FDh instruction.