Editing Zilog Z80 (section)

==== Undocumented instructions ====
The index registers, IX and IY, were intended as flexible 16-bit pointers, enhancing the ability to manipulate memory, stack frames and data structures. Officially, they were treated as 16-bit only. In reality they were implemented as a pair of 8-bit registers,<ref>{{Cite book |last=Froehlich |first=Robert A. |url=https://archive.org/details/freesoftwarecata0000froe/ |title=The free software catalog and directory |date=1984 |publisher=Crown Publishers |isbn=978-0-517-55448-7 |page=133 |quote=Undocumented Z80 codes allow 8 bit operations with IX and IY registers. |url-access=registration}}</ref> in the same fashion as the HL register, which is accessible either as 16 bits or separately as the ''H''igh and ''L''ow registers. The binary opcodes (machine language) were identical, but preceded by a new opcode prefix.<ref name="undocz80">{{Cite web |last=Bot |first=Jacco J. T. |title=Z80 Undocumented Instructions |url=http://www.z80.info/z80undoc.htm |url-status=live |archive-url=https://web.archive.org/web/20231223185034/http://z80.info/z80undoc.htm |archive-date=December 23, 2023 |website=Home of the Z80 CPU |quote=If an opcode works with the registers HL, H or L then if that opcode is preceded by #DD (or #FD) it works on IX, IXH or IXL (or IY, IYH, IYL), with some exceptions. The exceptions are instructions like LD H,IXH and LD L,IYH.}}</ref> Zilog published the opcodes and related mnemonics for the intended functions, but did not document the fact that every opcode that allowed manipulation of the H and L registers was equally valid for the 8 bit portions of the IX and IY registers. For example, the opcode 26h followed by an immediate byte value {{code|(LD H,n)}} will load that value into the H register. Preceding this two-byte instruction with the IX register's opcode prefix, DD, would instead result in the most significant 8 bits of the IX register being loaded with that same value. A notable exception to this would be instructions similar to {{code|LD H,(IX+d)}} which make use of both the HL and IX or IY registers in the same instruction;<ref name="undocz80" /> in this case the DD prefix is only applied to the (IX+d) portion of the instruction. The halves of the XY registers could also hold operands for 8-bit arithmetic, logical and compare instructions, sparing the regular 8-bit registers for other use. The undocumented ability to increment and decrement the upper half of an index register made it easy to expand the range of the normal indexed instructions, without having to resort to the documented {{code|ADD/SBC XY,DE}} or {{code|ADD/SBC XY,BC}}.

There are several other undocumented instructions as well.<ref>Robin Nixon ''The Amstrad Notepad Advanced User Guide'', Robin Nixon, 1993, {{ISBN|1-85058-515-6}}, pages 219–223.</ref> Undocumented or [[illegal opcode]]s are not detected by the Z80 and have various effects, some of which are useful. However, as they are not part of the formal definition of the instruction set, different implementations of the Z80 are not guaranteed (or especially likely) to work the same way for every undocumented opcode.