Z80 - undocumented opcodes - Revision history

DavidL: /* Web links */

2018-12-30T11:01:54Z

Web links

← Older revision		Revision as of 11:01, 30 December 2018
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	== Web links ==		== 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]]		[[Category:Programming]]

DavidL: /* Web links */

2018-12-30T10:59:59Z

Web links

← Older revision		Revision as of 10:59, 30 December 2018
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	*[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]
	*[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]]		[[Category:Programming]]

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.

← Older revision		Revision as of 10:14, 14 January 2013
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.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]]		[[Category:Programming]]

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.

← Older revision		Revision as of 10:05, 14 January 2013
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	== Web links ==		== 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]		*[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]]		[[Category:Programming]]

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

← Older revision		Revision as of 07:25, 10 December 2012
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	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.		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.		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.

	If you want a way to do an actual Shift Left Logical, ''i.e.'' ''register'' = ''register'' * 2, first ensure the carry is clear, inserting an OR A or AND A to achieve this if necessary, and then do an RL ''register''.

	<pre>CB30 SLL B		<pre>CB30 SLL B

Db6128: /* DD and FD prefixes */

2012-12-10T02:24:16Z

DD and FD prefixes

← Older revision		Revision as of 02:24, 10 December 2012
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	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.		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 -> LD IXH,A		<pre>DB #DD:LD H,A -> LD IXH,A

Db6128: /* DD and FD prefixes */

2012-12-10T02:23:17Z

DD and FD prefixes

← Older revision		Revision as of 02:23, 10 December 2012
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	== DD and FD prefixes ==		== 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.

			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.

	<pre>DB #DD:LD H,A -> LD IXH,A		<pre>DB #DD:LD H,A -> LD IXH,A
	DB #FD:LD B,L -> LD B,IYL		DB #FD:LD B,L -> LD B,IYL
	</pre>		</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 ==		== ED prefix ==

Db6128: /* CB prefix */

2012-12-10T02:13:56Z

CB prefix

← Older revision		Revision as of 02:13, 10 December 2012
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.		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 ~~represent~~ 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 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.

	Regardless of the story behind this operation, its effective result ~~of the operation~~ is ''register'' = (''register'' * 2) + 1, something that does have its uses and has been employed in various programming contexts for that reason.		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.

	If you want a way to do an actual Shift Left Logical, ''i.e.'' ''register'' = ''register'' * 2, first ensure the carry is clear, inserting an OR A or AND A to achieve this if necessary, and then do an RL ''register''.		If you want a way to do an actual Shift Left Logical, ''i.e.'' ''register'' = ''register'' * 2, first ensure the carry is clear, inserting an OR A or AND A to achieve this if necessary, and then do an RL ''register''.

Db6128: /* CB prefix */

2012-12-10T02:13:29Z

CB prefix

← Older revision		Revision as of 02:13, 10 December 2012
Line 7:		Line 7:
	== CB prefix ==		== 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.

			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 represent 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.

			Regardless of the story behind this operation, its effective result of the operation is ''register'' = (''register'' * 2) + 1, something that does have its uses and has been employed in various programming contexts for that reason.

			If you want a way to do an actual Shift Left Logical, ''i.e.'' ''register'' = ''register'' * 2, first ensure the carry is clear, inserting an OR A or AND A to achieve this if necessary, and then do an RL ''register''.

	<pre>CB30 SLL B		<pre>CB30 SLL B
	CB31 SLL C		CB31 SLL C
Line 16:		Line 23:
	CB36 SLL (HL)		CB36 SLL (HL)
	CB37 SLL A		CB37 SLL A
	</pre>		</pre>

	== DD and FD prefixes ==		== DD and FD prefixes ==

Skyhawk at 10:14, 10 September 2008

2008-09-10T10:14:58Z

← Older revision		Revision as of 10:14, 10 September 2008
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	Please note, that many Z80 successors like the Z180 are NOT able to execute some of the following opcodes properly.		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 ==		== CB prefix ==
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	== Web links ==		== 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]		*[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]]		[[Category:Programming]]