Changes
/* DD and FD prefixes */
The &DD or &FD prefixes are documented as causing operations using the 16-bit register HL to instead work with either of the 16-bit indexing registers IX or IY; if the operations access a memory location (''i.e.'' normally LD A,(HL), ''etc.''), the opcodes must additionally include an extra byte that specifies a signed displacement (-128 to +127) from IX/IY.
However, Zilog have not documented the fact that these prefixes also affect opcodes that usually refer to the 8-bit components of HL, ''i.e.'' H and L. Thus, one gains access to the additional registers IXH, IXL, IYH, and IYL, for almost all commands that normally use H or L. It is even possible to do things like LD IXH,IXL(although you cannot combine IX and IY in the same instruction, for obvious reasons). These registers can be useful in routines that must process and/or store a lot of numbers. Thankfully, they are not as slow as their 16-bit counterparts: whereas access to (IX+d) is usually slower by 3 NOPs than the equivalent operation upon (HL), using the 8-bit components is (like PUSH IX, ''etc.'') only 1 NOP slower, and this is only due to the need to parse the prefixing byte.
<pre>DB #DD:LD H,A -> LD IXH,A
