Z80 - undocumented opcodes - Revision history

DavidL: /* Web links */

2018-12-30T11:01:54Z

‎Web links

DavidL: /* Web links */

2018-12-30T10:59:59Z

‎Web links

FindYWay: /* Web links */ Fixed other broken link. Not 100% sure it's the same document.

2013-01-14T10:14:34Z

‎Web links: Fixed other broken link. Not 100% sure it's the same document.

FindYWay: /* Web links */ Corrected broken link. Instead link to official website.

2013-01-14T10:05:50Z

‎Web links: Corrected broken link. Instead link to official website.

Db6128: /* CB prefix */ removing one bit I wrote as I'm pretty sure it's nonsense and SLA works fine

2012-12-10T07:25:17Z

‎CB prefix: removing one bit I wrote as I'm pretty sure it's nonsense and SLA works fine

Deprecated: The each() function is deprecated. This message will be suppressed on further calls in /mnt/cpcwiki-httpdocs/includes/diff/DairikiDiff.php on line 434

Db6128: /* DD and FD prefixes */

2012-12-10T02:24:16Z

‎DD and FD prefixes

Db6128: /* DD and FD prefixes */

2012-12-10T02:23:17Z

‎DD and FD prefixes

Db6128: /* CB prefix */

2012-12-10T02:13:56Z

‎CB prefix

Db6128: /* CB prefix */

2012-12-10T02:13:29Z

‎CB prefix

Skyhawk at 10:14, 10 September 2008

2008-09-10T10:14:58Z

@@ Line 124: / Line 124: @@
 == Web links  ==
-*[http://www.myquest.nl/z80undocumented/z80-documented-v0.91.pdf "The Undocumented Z80 Documented" by Sean Young Version 0.91, 18th September, 2005] at [http://www.myquest.nl/z80undocumented/ Jan Wilmans' Website]
+* [http://www.myquest.nl/z80undocumented/z80-documented-v0.91.pdf "The Undocumented Z80 Documented" by Sean Young Version 0.91, 18th September, 2005] at [http://www.myquest.nl/z80undocumented/ Jan Wilmans' Website]
-*{{BrokenLink|http://www.grimware.org/lib/exe/fetch.php/documentations/devices/z80/z80.memptr.eng.txt|2017|01|22|21|32|33|text=Short information about the internal "MEMPTR" 16bit register of the Z80 and its influence on the F-Register}}
+* {{BrokenLink|http://www.grimware.org/lib/exe/fetch.php/documentations/devices/z80/z80.memptr.eng.txt|2017|01|22|21|32|33|text=Short information about the internal "MEMPTR" 16bit register of the Z80 and its influence on the F-Register}}
 [[Category:Programming]]

@@ Line 125: / Line 125: @@
 *[http://www.myquest.nl/z80undocumented/z80-documented-v0.91.pdf "The Undocumented Z80 Documented" by Sean Young Version 0.91, 18th September, 2005] at [http://www.myquest.nl/z80undocumented/ Jan Wilmans' Website]
-*[http://www.grimware.org/lib/exe/fetch.php/documentations/devices/z80/z80.memptr.eng.txt Short information about the internal "MEMPTR" 16bit register of the Z80 and its influence on the F-Register]
+*{{BrokenLink|http://www.grimware.org/lib/exe/fetch.php/documentations/devices/z80/z80.memptr.eng.txt|2017|01|22|21|32|33|text=Short information about the internal "MEMPTR" 16bit register of the Z80 and its influence on the F-Register}}
 [[Category:Programming]]

@@ Line 125: / Line 125: @@
 *[http://www.myquest.nl/z80undocumented/z80-documented-v0.91.pdf "The Undocumented Z80 Documented" by Sean Young Version 0.91, 18th September, 2005] at [http://www.myquest.nl/z80undocumented/ Jan Wilmans' Website]
-*[http://www.raww.org/index.php?name=News&file=article&sid=2226 Short information about the internal "MEMPTR" 16bit register of the Z80 and its influence on the F-Register]
+*[http://www.grimware.org/lib/exe/fetch.php/documentations/devices/z80/z80.memptr.eng.txt Short information about the internal "MEMPTR" 16bit register of the Z80 and its influence on the F-Register]
 [[Category:Programming]]

@@ Line 124: / Line 124: @@
 == Web links  ==
-*[http://www.robsy.net/z80undoc.pdf "The Undocumented Z80 Documented" by Sean Young at Jan Wilmans' Website]
+*[http://www.myquest.nl/z80undocumented/z80-documented-v0.91.pdf "The Undocumented Z80 Documented" by Sean Young Version 0.91, 18th September, 2005] at [http://www.myquest.nl/z80undocumented/ Jan Wilmans' Website]
 *[http://www.raww.org/index.php?name=News&file=article&sid=2226 Short information about the internal "MEMPTR" 16bit register of the Z80 and its influence on the F-Register]
 [[Category:Programming]]

@@ Line 9: / Line 9: @@
 Of the 247 opcodes that use the prefix &CB, the block &CB &30 to &CB &37 is undocumented officially. These commands shift the operand register left and set its lowest bit to 1.
-This is in contrast to SRL (Shift Right Logical), which shifts right and clears the highest bit. Some believe that this difference indicates why these opcodes are undocumented, feeling that this result represents a bug in the Z80 and that the operation was supposed to be Shift Left Logical, which would thus ''clear'' the lowest bit after shifting. Others call the opcodes SLIA, for Shift Left Inverted Arithmetic.
+This is in contrast to SRL (Shift Right Logical), which shifts right and clears the highest bit. Some believe these opcodes were supposed to be Shift Left Logical but that the setting of the lowest bit represents a bug in the Z80, claiming this is why the opcodes are undocumented. Others call the opcodes SLIA, for Shift Left Inverted Arithmetic.
 Regardless of the story behind this operation, its effective result is ''register'' = (''register'' * 2) + 1, something that does have its uses and has been employed in various programming contexts for that reason.
 <pre>CB30 SLL B

@@ Line 29: / Line 29: @@
 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. 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.
+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 -&gt; LD IXH,A

@@ Line 27: / Line 27: @@
 == DD and FD prefixes  ==
-The DD/FD prefixes replace the registers L, H or HL with IXL/IYL, IXH/IYH or IX/IY.
+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.
 <pre>DB #DD:LD H,A -&gt; LD IXH,A
 DB #FD:LD B,L -&gt; LD B,IYL
 </pre>
-Note: These registers are called LX, LY, HX and HY in the WinAPE assembler.
+Note: These registers are called LX, LY, HX, and HY by [[WinAPE]]'s debugger, although its assembler uses the names given above.
 == ED prefix  ==

@@ Line 7: / Line 7: @@
 == CB prefix  ==
-The following commands set the lowest bit to 1 instead to 0:
+Of the 247 opcodes that use the prefix &CB, the block &CB &30 to &CB &37 is undocumented officially. These commands shift the operand register left and set its lowest bit to 1.
 <pre>CB30 SLL B
 CB31 SLL C
@@ Line 16: / Line 23: @@
 CB36 SLL (HL)
 CB37 SLL A
 </pre>
 == DD and FD prefixes  ==

@@ Line 3: / Line 3: @@
 Please note, that many Z80 successors like the Z180 are NOT able to execute some of the following opcodes properly.
-This is just an overview. Parts of this article have been copied from the [http://www.robsy.net/z80undoc.pdf "The Undocumented Z80 Documented" originally by Sean Young] and currently maintained by Jan Wilmans, which is one of the most comprehensive descriptions around.
+This is just an overview. Parts of this article have been copied from the [http://www.myquest.nl/z80undocumented/ "The Undocumented Z80 Documented"] originally by Sean Young and currently maintained by Jan Wilmans, which is one of the most comprehensive descriptions around.
 == CB prefix  ==
@@ Line 115: / Line 115: @@
 == Web links  ==
-*[http://www.robsy.net/z80undoc.pdf "The Undocumented Z80 Documented" by Sean Young at Robsy's MSX Workshop]
+*[http://www.robsy.net/z80undoc.pdf "The Undocumented Z80 Documented" by Sean Young at Jan Wilmans' Website]
 *[http://www.raww.org/index.php?name=News&file=article&sid=2226 Short information about the internal "MEMPTR" 16bit register of the Z80 and its influence on the F-Register]
 [[Category:Programming]]