View source for Halt and Catch Fire (computing)

{{Short description|Computer machine code instruction}}
{{For|the TV series|Halt and Catch Fire (TV series)}}{{For|the Supernatural episode|Supernatural season 10}}
{{Use dmy dates|date=July 2020|cs1-dates=y}}
{{Use list-defined references|date=June 2022}}

In computer engineering, '''Halt and Catch Fire''', known by the [[Assembly language#Opcode mnemonics and extended mnemonics|assembly language mnemonic]] '''HCF''', is an [[idiom]] referring to a computer [[machine code]] [[Instruction (computer science)|instruction]] that causes the computer's [[central processing unit]] (CPU) to cease meaningful operation, typically requiring a restart of the computer. It originally referred to a fictitious instruction in [[IBM System/360 architecture|IBM System/360]] computers (introduced in 1964), making a joke about its numerous non-obvious instruction mnemonics.

After the introduction of the [[MC6800]] processor in 1974, a design flaw was discovered by programmers. Due to incomplete opcode decoding, two [[illegal opcode]]s, 0x9D and 0xDD, will cause the [[program counter]] on the processor to increment endlessly, which locks the processor until reset. Those codes have been unofficially named HCF. During the design process of the MC6802, engineers originally planned to remove this instruction, but kept it as-is for testing purposes. As a result, HCF was officially recognized as a real instruction.<ref name="Bryan_2021" /><ref name="Daniels_1985" /> Later, HCF became a humorous catch-all term for instructions that may freeze a processor, including intentional instructions for testing purposes, and unintentional illegal instructions. Some are considered hardware defects, and if the [[Multi-user software|system is shared]], a malicious user can execute them to launch a [[denial-of-service attack]].

In the case of real instructions, the implication of this expression is that, whereas in most cases in which a CPU executes an unintended instruction (a bug in the code) the computer may still be able to recover, in the case of an HCF instruction there is, by definition, no way for the system to recover without a restart.

The expression ''catch fire'' is a facetious exaggeration of the speed with which the CPU chip would be switching some bus circuits, purportedly causing them to overheat and burn.<ref name="Jargon_2012" />

== Origins ==
The [[Z1 (computer)|Z1]] (1938) and [[Z3 (computer)|Z3]] (1941) computers built by [[Konrad Zuse]] contained illegal sequences of instructions which damaged the hardware if executed by accident.<ref name="Rojas_1997" />

Apocryphal stories connect this term with an [[illegal opcode]] in [[IBM System/360 architecture|IBM System/360]]. A processor, upon encountering the instruction, would start switching [[Bus (computing)|bus lines]] very fast, potentially leading to overheating.<ref name="Clements_2006" /><ref name="Kohler_2005" />

In a computer's [[assembly language]], mnemonics are used that are directly equivalent to [[machine code]] instructions. The mnemonics are frequently three letters long, such as ADD, CMP (to compare two numbers), and JMP (jump to a different location in the program). The HCF instruction was originally a fictitious assembly language instruction, said to be under development at [[IBM]] for use in their [[IBM System/360 architecture|System/360]] computers, along with many other amusing [[three-letter acronym]]s like XPR (Execute Programmer) and CAI (Corrupt Accounting Information), and similar to other joke mnemonics such as "SDI" for "Self Destruct Immediately"<ref name="Dunlap_2016" /> and "CRN" for "Convert to Roman Numerals".<ref name="Cirsovius_2015" /> A list of such mnemonics, including HCF, shows up as "Overextended Mnemonics" in the April 1980 ''[[Creative Computing]]'' flip-side parody issue.<ref name="CC_1980" />

== In modern CPUs ==
CPU designers sometimes incorporate one or more undocumented machine code instructions for testing purposes, such as the IBM System/360 DIAGnose instruction.<ref name="IBM360_2012" />

=== Motorola 6800 ===
The [[Motorola 6800]] [[microprocessor]] was the first for which an undocumented assembly mnemonic HCF became widely known. The operation codes ([[opcode]]s—the portions of the machine language instructions that specify an operation to be performed) hexadecimal 9D and DD were reported and given the unofficial mnemonic HCF in a December 1977 article by Gerry Wheeler in [[Byte (magazine)|''BYTE'' magazine]] on undocumented opcodes.<ref name="Wheeler_1977" /> Wheeler noted that Motorola reported 197 valid operation codes for the M6800 processor, and so inferred that with 256 possible 8 bit combinations, there must be 59 invalid instructions. He described the HCF as a "big surprise", and said of the Catch Fire portion of the moniker, "Well, almost":

{{quotation|1=When this instruction is run the only way to see what it is doing is with an [[oscilloscope]]. From the user's point of view the machine halts and defies most attempts to get it restarted. Those persons with indicator lamps on the [[address bus]] will see that the processor begins to read all of the memory, sequentially, very quickly. In effect, the address bus turns into a 16 bit counter. However, the processor takes no notice of what it is reading... it just reads.<ref name="Wheeler_1977" />}}

Another author wrote in 2002:

{{quote|In the old days of the Motorola 6800 microprocessor, instruction code DD caused the processor to go into an endless loop, reading from each memory address in order. (Other engineers referred to this as the "Halt and Catch Fire" [HCF] instruction, but we remembered the code by calling it the "Drop Dead" instruction.) Drop Dead mode was wonderful for spotting hardware timing and address logic problems with a scope; all of the address and clock lines were nice, cycling square waves.<ref name="Agans_2002" />}}

The 6800's behavior when encountering HCF was known to Motorola by 1976. When the 6800 encounters the HCF instruction, the processor never finds the end of it, endlessly incrementing its program counter until the CPU is reset.<ref name="MC6800_HCF_Explanation">{{cite journal |last1=Daniels |first1=R. Gary |last2=Bruce |first2=William |title=Built-In Self-Test Trends in Motorola Microprocessors |journal=IEEE Design & Test of Computers |date=April 1985 |volume=2 |issue=2 |page=64 |doi=10.1109/MDT.1985.294865 |bibcode=1985IDTC....2...64D |s2cid=22719798 }}</ref> Hence, the [[address bus]] effectively becomes a [[Counter (digital)|counter]], allowing the operation of all [[address line]]s to be quickly verified. Once the processor entered this mode, it is not responsive to [[interrupt]]s, so normal operation can only be restored by a reset (hence the "Drop Dead" and "Halt and Catch Fire" monikers). These references are thus to the unresponsive behavior of the CPU in this state, and not to any form of erratic behavior.{{citation needed|date=October 2015}} Motorola kept the HCF behavior in the 6802 variant of the processor (which released in 1977) as an intentional self-test for the 6802's 128 bytes of onboard RAM.

Other HCF-like instructions were found later on the Motorola 6800 when executing undocumented opcodes FD (cycling twice slower than 9D/DD) or CD/ED (cycling at a human-readable very low frequency on a limited number of high-address lines).<ref name="Demeulemeester_2019" />

HCF is believed to be the first built-in self-test feature on a Motorola microprocessor.<ref name="Daniels_1985" />

=== Intel x86 ===
{{See also|HLT (x86 instruction)|Pentium F00F bug}}

The Intel 8086 and subsequent processors in the x86 series have an [[HLT (x86 instruction)|HLT]] (halt) instruction, opcode F4, which stops instruction execution and places the processor in a HALT state. An enabled interrupt, a debug exception, the BINIT signal, the INIT signal, or the RESET signal resumes execution, which means the processor can always be restarted.<ref name="Jeschke_2014" /> Some of the early Intel DX4 chips have a problem with the HLT instruction and cannot be restarted after this instruction is used, which disables the computer and turns HLT into more of an HCF instruction. The Linux kernel has a "no-hlt" option telling Linux to run an infinite loop instead of using HLT, which allows users of these broken chips to use Linux.<ref name="Gortmaker_2003" />

The 80286 has the undocumented opcode 0F 04, causing the CPU to hang when executed. The only way out is a CPU reset.{{Citation needed|date=August 2017}}<ref name="Sandpile_2004" /> In some implementations, the opcode is emulated through [[BIOS]] as a [[HLT (x86 instruction)|halt]]ing sequence.<ref name="Sandpile_2003" />

Many computers in the Intel Pentium line can be locked up by executing an invalid instruction (F00F C7C8), which causes the computer to lock up. This became known as the [[Pentium F00F bug]]. No compiler creates the instruction, but a malicious programmer can insert it into code to render an afflicted computer inoperable until the machine is [[Power cycling|power-cycled]]. Since its discovery, workarounds have been developed to prevent it from locking the computer, and the bug has been eliminated in subsequent Intel processors.<ref name="Collins_1998" /><ref name="Intel_1999" />

During [[Black Hat Briefings|Black Hat USA 2017]], Christopher Domas showed that he found a new "Halt and Catch Fire" instruction<ref name="Domas_2017_1" /><ref name="Domas_2017_2" /> on an [[Responsible disclosure|undisclosed]] x86 processor model using his own x86 processor [[Fuzzing|fuzzer]] called sandsifter.<ref name="Domas_2017_3" />

=== Other CPUs ===
The [[NMOS logic|NMOS]] [[MOS Technology 6502]] has 12 invalid instructions which cause the [[program counter]] to fail to fetch the next instruction, locking up the CPU and requiring a processor reset.<ref name="Steil_2016" /><ref name="Offenga_2016" />  The [[Western Design Center|WDC]] version of the [[CMOS]] [[65C02]], as well as the [[65C816]], has the <code>STP</code> (stop, opcode <code>$DB</code>) instruction.  When executed, <code>STP</code> will stop the processor's internal clock, causing all processing to cease—also, the processor will be unresponsive to all inputs except <code>RESB</code> (reset).  The only way to clear the effects of an <code>STP</code> instruction is to toggle <code>RESB</code>.

On the [[Zilog Z80]], executing DI (disable interrupts) followed by HALT (wait for an interrupt) results in the CPU staying frozen indefinitely, waiting for an interrupt that cannot happen. However, the non-maskable interrupt signal can be used to break out of this state, making this pair not a true HCF.<ref name="SMS_2016" /><ref name="Flammenkamp_2016" /> The {{nowrap|/NMI}} signal is on Pin 17 of the original 40 pin DIP package.<ref name="Z80_2016" /><ref name="Vis_2016" /> The pair will only result in a HCF condition if either the {{nowrap|/NMI}} pin is connected directly to the +5V rail, making the generation of that signal impossible, or if the interrupt routine that services {{nowrap|/NMI}} ends with a return, placing it back in the HALT state.

The SM83 processor{{efn|The SM83 CPU is similar to the Z80, but not directly related.}}<ref name="Gameboy_SM83" /> core in the [[Game Boy]]'s LR35902 system on chip has a similar issue, triggered by two consecutive HALTs with interrupts disabled.{{efn|When interrupts are disabled, the HALT instruction on the Game Boy CPU does not pause the CPU, but, rather, prevents the CPU's program counter from incrementing on the instruction immediately following the HALT, effectively doubling the instruction after the HALT (or, for a multi-byte instruction, doubling the first byte and separating off the original last byte into a new single-byte instruction); if the instruction after the HALT is itself a HALT, then (as HALT is a single-byte instruction) the CPU effectively sees an infinite series of HALTs, causing the system to lock up.}}<ref name="Gameboy_2018" /> The core itself contains 11 opcodes that fully lock the CPU when executed.<ref name="Gameboy_2021" />

The SC61860, mainly used in [[Sharp Corporation|Sharp]] [[pocket computer]]s in the 1980s and 1990s, has an undocumented HCF instruction with the opcode 7B.<ref name="ESR_2022" />

== See also ==
* [[Cyrix coma bug]]
* [[Pentium F00F bug]]
* [[Killer poke]]
* [[Magic smoke]]
* [[lp0 on fire]]
* [[Write-only memory (joke)]]
* [[NOP (code)]]

== Notes ==
{{Notelist}}

== References ==
{{reflist|refs=
<ref name="Dunlap_2016">{{cite web |title=A Proposed Instruction Set |author-last=Dunlap |author-first=Bryan |website=Physics Department, The Ohio State University |url=https://www.physics.ohio-state.edu/~bcd/humor/instruction.set.html |access-date=2016-06-20 |url-status=dead |archive-url=https://web.archive.org/web/20170908154113/https://www.physics.ohio-state.edu/~bcd/humor/instruction.set.html |archive-date=2017-09-08}}</ref>
<ref name="IBM360_2012">{{cite book |title=IBM System/360 Principles of Operation |publisher=[[IBM]] |url=http://bitsavers.org/pdf/ibm/360/princOps/A22-6821-0_360PrincOps.pdf |access-date=2014-07-02 |url-status=live |archive-date=2012-02-29 |archive-url=https://web.archive.org/web/20120229200257/http://bitsavers.org/pdf/ibm/360/princOps/A22-6821-0_360PrincOps.pdf}}</ref>
<ref name="Wheeler_1977">{{Cite magazine |date=December 1977 |title=Undocumented M6800 Instructions |url=https://archive.org/details/byte-magazine-1977-12/page/n47/mode/2up?view=theater |magazine=[[BYTE]] |volume=2 |issue=12 |pages=46–47 |access-date=2023-11-20 |quote="The mnemonics are, of course, assigned by me." |author-last=Wheeler |author-first=Gerry}}</ref>
<ref name="Daniels_1985">{{Cite journal |title=Built-In Self-Test Trends in Motorola Microprocessors |author-last1=Daniels |author-first1=R. Gary |author-last2=Bruce |author-first2=William |journal=IEEE Design & Test |volume=2 |issue=2 |pages=64–71 |date=April 1985 |doi=10.1109/MDT.1985.294865 |bibcode=1985IDTC....2...64D |s2cid=22719798 |quote=To add insult to injury, we discovered that we had an illegal HACOF, an instruction that our customers found on the MC6800. It was an unused opcode-an illegal instruction. When executed inadvertently, the program counter would increment indefinitely. The problem, which was caused by incomplete opcode decoding, was a nuisance because Reset was the only means of terminating the instruction. ... During the design process, we figured out how to eliminate the HACOF instruction. About that time, the product engineers came to us with an idea. They said, 'You know what we'd really like? Some way to quickly test the RAM. If we could somehow point the program counter at the first RAM address and then just increment through the RAM, we could test it a lot faster.' Since the HACOF 'instruction' did precisely that—and we really didn't want to invest the effort needed to remove it—we replied, 'Have we got a deal for you!' HACOF thus became the first intentional built-in self-test feature on a Motorola microprocessor.}}</ref>
<ref name="Collins_1998">{{cite web |title=The Pentium F00F Bug: Workarounds for a nasty problem |author-last=Collins |author-first=Robert R. |date=1998-05-01 |publisher=[[Dr. Dobb's Journal]] |url=http://www.ddj.com/architect/184410555 |access-date=2014-08-12 |url-status=live |archive-url=https://web.archive.org/web/20220430005211/https://www.drdobbs.com/embedded-systems/the-pentium-f00f-bug/184410555 |archive-date=2022-04-30}}</ref>
<ref name="Intel_1999">{{cite book |title=Pentium Processor Specification Update |publisher=[[Intel Corporation]] |date=January 1999 |id=Order number 242480-041 |pages=51–52 |url=http://download.intel.com/support/processors/pentium/sb/242480.pdf |access-date=2006-11-02 |url-status=live |archive-date=2016-03-04 |archive-url=https://web.archive.org/web/20160304060905/http://download.intel.com/support/processors/pentium/sb/242480.pdf}}</ref>
<ref name="Bryan_2021">{{cite web |title=6800 Instruction Set |website=Bryan's Old Computers |url=http://bytecollector.com/archive/digital_group/documentation/hardware/dg_systems/system_cards/6800_cpu_card/6800_Instruction_Set.PDF |access-date=2022-04-09 |url-status=live |archive-url=https://web.archive.org/web/20210501023253/http://www.bytecollector.com/archive/digital_group/documentation/hardware/dg_systems/system_cards/6800_cpu_card/6800_Instruction_Set.PDF |archive-date=2021-05-01}}</ref>
<ref name="Jargon_2012">{{Cite web |title=Jargon File entry for the HCF assembly mnemonic |url=http://www.catb.org/jargon/html/H/HCF.html |access-date=2014-05-04 |url-status=live |archive-url=https://web.archive.org/web/20120520113656/http://catb.org/jargon/html/H/HCF.html |archive-date=2012-05-20}}</ref>
<ref name="Clements_2006">{{cite conference |title=Embedding Ethics in Computer Architecture |author-last1=Clements |author-first1=Alan |date=2006-10-28 |edition=36 |conference=ASEE/IEEE Frontiers in Education Conference |page=4 |url=https://www.researchgate.net/publication/224061061 |access-date=2018-03-02 |url-status=live |archive-url=https://web.archive.org/web/20220430005210/https://www.researchgate.net/publication/224061061_Embedding_Ethics_in_Computer_Architecture |archive-date=2022-04-30}}</ref>
<ref name="Kohler_2005">{{cite web |title=CS111 - Lecture 1 |author-last=Kohler |author-first=Eddie |author-link=Eddie Kohler |date=2005-04-04 |page=2 |url=http://www.read.seas.harvard.edu/~kohler/class/cs111-s05/notes/notes1.pdf |access-date=2018-03-02 |url-status=live |archive-url=https://web.archive.org/web/20180302164550/http://www.read.seas.harvard.edu/~kohler/class/cs111-s05/notes/notes1.pdf |archive-date=2018-03-02}}</ref>
<ref name="Cirsovius_2015">{{cite web |title=Far out op codes |author-first=Werner |author-last=Cirsovius |url=http://www.cirsovius.de/Firmen/Uni-Chaos/FUN/opcodes.html |access-date=2015-05-28 |url-status=dead |archive-url=https://web.archive.org/web/20160305150047/http://www.cirsovius.de/Firmen/Uni-Chaos/FUN/opcodes.html |archive-date=2016-03-05}}</ref>
<ref name="CC_1980">{{cite journal |title=Overextended Mnemonics |date=April 1980 |journal=[[Creative Computing]] |volume=6 |issue=4 |pages=17 (hex) (flip–side) |url=https://archive.org/stream/creativecomputing-1980-04/Creative_Computing_v06_n04_1980_Apr#page/n205/mode/2up |access-date=2017-03-12}}</ref>
<ref name="Agans_2002">{{cite book |title=Debugging: the 9 indispensable rules for finding even the most elusive software and hardware problems |author-last=Agans |author-first=David J. |date=2002 |publisher=American Management Association |location=New York, USA |isbn=978-0-81442678-4 |oclc=52043345 |page=77 |url=https://books.google.com/books?id=jynA9ECbBsgC&pg=PA77 |access-date=2016-10-30 |url-status=live |archive-url=https://web.archive.org/web/20140726162952/http://books.google.com/books?id=jynA9ECbBsgC&pg=PA77 |archive-date=2014-07-26}}</ref>
<ref name="Demeulemeester_2019">{{cite web |title=Investigating the HCF (Halt & Catch Fire) instruction on Motorola 6800 |author-last=Demeulemeester |author-first=Samuel |date=2019-07-17 |website=X86.FR – Doc TB's R&D Lab |language=en-US |url=https://x86.fr/investigating-the-halt-and-catch-fire-instruction-on-motorola-6800/ |access-date=2022-04-09 |url-status=live |archive-url=https://web.archive.org/web/20220331005900/https://x86.fr/investigating-the-halt-and-catch-fire-instruction-on-motorola-6800/ |archive-date=2022-03-31}}</ref>
<ref name="Jeschke_2014">{{cite web |title=x86 Instruction Set Reference: HLT |url=http://x86.renejeschke.de/html/file_module_x86_id_134.html |access-date=2014-07-02 |url-status=live |archive-url=https://web.archive.org/web/20140714162407/http://x86.renejeschke.de/html/file_module_x86_id_134.html |archive-date=2014-07-14}}</ref>
<ref name="Gortmaker_2003">{{cite web |title=The Linux Boot Prompt-How To |author-last=Gortmaker |author-first=Paul |date=2003-03-21 |publisher=The Linux Documentation Project |url=http://www.tldp.org/HOWTO/pdf/BootPrompt-HOWTO.pdf |access-date=2014-07-02 |url-status=live |archive-url=https://web.archive.org/web/20150706054138/http://www.tldp.org/HOWTO/pdf/BootPrompt-HOWTO.pdf |archive-date=2015-07-06}}</ref>
<ref name="Sandpile_2004">{{cite web |url=http://www.sandpile.org/post/msgs/20004129.htm |archive-url=https://web.archive.org/web/20041106070621/http://www.sandpile.org/post/msgs/20004129.htm |title=Re: Undocumented opcodes (HINT_NOP) |archive-date=2004-11-06 |access-date=2010-11-07}}</ref>
<ref name="Sandpile_2003">{{cite web |url=http://www.sandpile.org/post/msgs/20003986.htm |archive-url=https://web.archive.org/web/20030626044017/http://www.sandpile.org/post/msgs/20003986.htm |title=Re: Also some undocumented 0Fh opcodes |archive-date=2003-06-26 |access-date=2010-11-07}}</ref>
<ref name="Domas_2017_1">{{cite web |url=https://www.blackhat.com/docs/us-17/thursday/us-17-Domas-Breaking-The-x86-ISA.pdf |title=Breaking the x86 ISA (PDF) |publisher=Christopher Domas |access-date=2017-12-09 |archive-date=2018-01-04 |archive-url=https://web.archive.org/web/20180104124624/https://www.blackhat.com/docs/us-17/thursday/us-17-Domas-Breaking-The-x86-ISA.pdf |url-status=live}}</ref>
<ref name="Domas_2017_2">{{cite web |url=https://www.youtube.com/watch?v=KrksBdWcZgQ |archive-url=https://ghostarchive.org/varchive/youtube/20211221/KrksBdWcZgQ |archive-date=2021-12-21 |url-status=live |title=Breaking the x86 ISA (video) |date=31 August 2017 |publisher=Christopher Domas |access-date=2017-12-09}}{{cbignore}}</ref>
<ref name="Domas_2017_3">{{cite web |url=https://github.com/xoreaxeaxeax/sandsifter |title=sandsifter: the x86 processor fuzzer |publisher=Christopher Domas |access-date=2017-12-09 |archive-date=2017-10-25 |archive-url=https://web.archive.org/web/20171025202112/https://github.com/xoreaxeaxeax/sandsifter |url-status=live}}</ref>
<ref name="Steil_2016">{{cite web |author-last=Steil |author-first=Michael |title=How MOS 6502 Illegal Opcodes really work |url=http://www.pagetable.com/?p=39 |website=pagetable.com |access-date=2016-08-01 |archive-date=2016-07-07 |archive-url=https://web.archive.org/web/20160707220443/http://www.pagetable.com/?p=39 |url-status=live}}</ref>
<ref name="Offenga_2016">{{cite web |author-last=Offenga |author-first=Freddy |title=6502 Undocumented Opcodes |url=http://nesdev.com/undocumented_opcodes.txt |website=NesDev |access-date=2016-08-01 |archive-date=2016-08-08 |archive-url=https://web.archive.org/web/20160808053007/http://nesdev.com/undocumented_opcodes.txt |url-status=live}}</ref>
<ref name="Gameboy_2018">{{cite web |url=http://marc.rawer.de/Gameboy/Docs/GBCPUman.pdf#page=21 |title=GameBoy CPU Manual |access-date=2018-06-22 |archive-date=2018-06-23 |archive-url=https://web.archive.org/web/20180623060718/http://marc.rawer.de/Gameboy/Docs/GBCPUman.pdf#page=21 |url-status=live}}</ref>
<ref name="Gameboy_2021">{{cite web |url=https://gbdev.io/gb-opcodes/optables/ |title=Game Boy CPU instruction set |access-date=2021-03-11 |archive-date=2021-02-09 |archive-url=https://web.archive.org/web/20210209024452/https://gbdev.io/gb-opcodes/optables/ |url-status=live}}</ref>
<ref name="Gameboy_SM83">{{cite web |url=https://github.com/Gekkio/gb-research/tree/main/sm83-cpu-core |title=Game Boy SM83 CPU reverse engineering |website=[[GitHub]] |access-date=2022-11-08 |archive-date=2022-10-29 |archive-url=https://web.archive.org/web/20221029214709/https://github.com/Gekkio/gb-research/tree/main/sm83-cpu-core |url-status=live}}</ref>
<ref name="SMS_2016">{{cite web |url=http://www.smspower.org/Development/InterruptMechanism |title=Interrupt Mechanism - Development - SMS Power! |access-date=2016-04-25 |archive-date=2016-04-04 |archive-url=https://web.archive.org/web/20160404071536/http://www.smspower.org/Development/InterruptMechanism |url-status=live}}</ref>
<ref name="Flammenkamp_2016">{{cite web |url=http://www.z80.info/interrup.htm |title=Interrupt Behaviour of the Z80 CPU |author-first=Achim |author-last=Flammenkamp |access-date=2016-04-25 |archive-date=2016-04-20 |archive-url=https://web.archive.org/web/20160420035105/http://www.z80.info/interrup.htm |url-status=live}}</ref>
<ref name="Z80_2016">{{cite web |url=http://www.cpu-world.com/info/Pinouts/Z80.html |title=Pinouts - Z80 family |access-date=2016-04-25 |archive-date=2016-05-08 |archive-url=https://web.archive.org/web/20160508044002/http://www.cpu-world.com/info/Pinouts/Z80.html |url-status=live}}</ref>
<ref name="Vis_2016">{{cite web |url=http://www.petervis.com/electronics/CPU_Processors/Zilog_Z80A/Z80A_CPU_Features.html |title=Zilog Z80 Pinout |author-first=Peter J. |author-last=Vis |access-date=2016-04-25 |archive-date=2016-10-11 |archive-url=https://web.archive.org/web/20161011173632/http://www.petervis.com/electronics/CPU_Processors/Zilog_Z80A/Z80A_CPU_Features.html |url-status=live}}</ref>
<ref name="ESR_2022">{{cite web |url=https://github.com/utz82/SC61860-Instruction-Set |title=SC61860 (Aka ESR-H) Instruction Set |website=[[GitHub]] |date=2022-03-20 |access-date=2022-03-23 |archive-date=2022-03-23 |archive-url=https://web.archive.org/web/20220323131202/https://github.com/utz82/SC61860-Instruction-Set |url-status=live}}</ref>
<ref name="Rojas_1997">{{cite journal |title=Konrad Zuse's Legacy: The Architecture of the Z1 and Z3 |author-last=Rojas |author-first=Raúl |author-link=Raúl Rojas |journal=[[IEEE Annals of the History of Computing]] |volume=19 |number=2 |date=April–June 1997 |pages=5–16 [9–10] |doi=10.1109/85.586067 |url=http://ed-thelen.org/comp-hist/Zuse_Z1_and_Z3.pdf |access-date=2022-07-03 |url-status=live |archive-url=https://web.archive.org/web/20220703082408/http://ed-thelen.org/comp-hist/Zuse_Z1_and_Z3.pdf |archive-date=2022-07-03 |quote-page=10 |quote=There are a lot of details that the engineer designing the "microprogram" must keep in mind, otherwise short circuits can destroy the hardware. The [[Z1 (computer)|Z1]] with its mechanical design was still more sensitive in this respect than the [[Z3 (computer)|Z3]]. Even after it was completed, there were sequences of instructions that the programmer had to avoid in order not to damage the hardware. One of those sequences was inadvertently tried at the [[Berlin Museum of Technology and Transportation]], which led to slight damaging of the reconstructed Z1 in 1994.}} (12 pages)</ref>
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{{Error messages}}

[[Category:Computer errors]]
[[Category:Computer folklore]]
[[Category:Computer humour]]
[[Category:Hardware bugs]]
[[Category:Machine code]]