Microcode - Revision history

Jnc: /* Further reading */ Add re-print cite

2026-10-16T08:49:27Z

Jnc: Allows loops; not the only use; typos

2025-12-25T07:15:40Z

Allows loops; not the only use; typos

← Older revision		Revision as of 07:15, 25 December 2025
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	'''Microcode''' (in the form of a '''microprogram''') refers to a technique for implementing the functionality needed to control the internal operation of a [[Central Processing Unit\|CPU]].		'''Microcode''' (in the form of a '''microprogram''') refers to a technique for implementing the functionality needed to control the internal operation of a [[Central Processing Unit\|CPU]].

	On each '''microcycle''' (analogous, internally, to the cycles of the main CPU), a '''microinstruction''' is [[fetch]]ed, and used to control the operation of the CPU's internal [[hardware]] elements during that microcycle: routing data out of [[register]]s (including internal registers not visible to the programmer) and/or [[main memory]], through the [[Arithmetic logic unit\|ALU]] (while selecting which [[operator\|operation]] is to be performed by it), and then storing the results as ~~apropriate~~. They are thus ~~analagous~~ to the [[instruction]]s of the main CPU, but at a much lower level.		On each '''microcycle''' (analogous, internally, to the cycles of the main CPU), a '''microinstruction''' is [[fetch]]ed, and used to control the operation of the CPU's internal [[hardware]] elements during that microcycle: routing data out of [[register]]s (including internal registers not visible to the programmer) and/or [[main memory]], through the [[Arithmetic logic unit\|ALU]] (while selecting which [[operator\|operation]] is to be performed by it), and then storing the results as appropriate. They are thus analogous to the [[instruction]]s of the main CPU, but at a much lower level.

	==Background==		==Background==
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	==Enhancements==		==Enhancements==

	In 1951, [[Maurice Wilkes]] suggested more widespread use of that technique (to get rid of the design burden, and chance of errors, in using un-structured logic aggregations for control); he added [[conditional branch]]ing, to make microcode more flexible. Over time, more concepts from [[higher-level language]]s have been added to microcode, such as [[subroutine]]s.		In 1951, [[Maurice Wilkes]] suggested more widespread use of that technique (to get rid of the design burden, and chance of errors, in using un-structured logic aggregations for control); he added [[conditional branch]]ing (which also allowed [[loop]]ing), to make microcode more flexible. Over time, more concepts from [[higher-level language]]s have been added to microcode, such as [[subroutine]]s.

	Although early microcode was implemented in read-only memory (the Whirlwind ROM was a [[diode]] matrix), some later computers included high-speed [[Random Access Memory\|RAM]] for part of the control store, so that the instruction set could be dynamically extended to speed up applications.		Although early microcode was implemented in read-only memory (the Whirlwind ROM was a [[diode]] matrix), some later computers included high-speed [[Random Access Memory\|RAM]] for part of the control store, so that the instruction set could be dynamically extended to, for example, speed up applications.

	Recently, microcode has been augmented with [[programmable logic array]]s, which are usually given the job of decoding instructions (something done more efficiently in combinatorial logic).		Recently, microcode has been augmented with [[programmable logic array]]s, which are usually given the job of decoding instructions (something done more efficiently in combinatorial logic).

Jnc: /* Another meaning */ Old DEC meaning of 'microcoding'

2025-07-16T16:53:12Z

Another meaning: Old DEC meaning of 'microcoding'

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	Once a microcoded CPU had been designed, it was relatively easy to expand the [[instruction set]] to include more complex instructions (something whose only per-unit repeated cost was larger microcode memories). For much of the lifetime of computers, the CPU was faster than the main memory; in such an environment, it made sense to make the instructions of the CPU as powerful (complicated) as possible, so a single instruction fetch (which is, of course, pure overhead in terms of the actual computation) does as much as possible. Such complex instructions are more easily implemented with a microcode approach, than with combinatorial logic.		Once a microcoded CPU had been designed, it was relatively easy to expand the [[instruction set]] to include more complex instructions (something whose only per-unit repeated cost was larger microcode memories). For much of the lifetime of computers, the CPU was faster than the main memory; in such an environment, it made sense to make the instructions of the CPU as powerful (complicated) as possible, so a single instruction fetch (which is, of course, pure overhead in terms of the actual computation) does as much as possible. Such complex instructions are more easily implemented with a microcode approach, than with combinatorial logic.

			==Another meaning==

			Prior to the modern term/concept, 'microcoding' had another fairly common (but totally unrelated) meaning. Old [[instruction set architecture\|ISA's]] often had instructions where a single [[operation code]] could accomplish many different low-level operations (such as clearing various registers, and clearing or setting particular [[condition codes\|condition code]] bits). Each possible low-level operation was assigned a particular bit in the instruction word; setting more than one of those bits caused all of the selected low-level operations to be performed. Doing so as often termed 'microcoding'. Some combinations were quite useful (and occasionally were given their own names in the [[assembly language]] for that machine); see [[PDP-8 architecture#Group 1 "operate" instruction operations\|here]] for an example).

	==Further reading==		==Further reading==
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	* M. V. Wilkes, [https://www.cs.princeton.edu/courses/archive/fall10/cos375/BestWay.pdf The Best Way to Design an Automatic Computing Machine], Report of Manchester University Computer Inaugural Conference, July 1951, pp. 16–18 - Wilkes' original note on microcode		* M. V. Wilkes, [https://www.cs.princeton.edu/courses/archive/fall10/cos375/BestWay.pdf The Best Way to Design an Automatic Computing Machine], Report of Manchester University Computer Inaugural Conference, July 1951, pp. 16–18 - Wilkes' original note on microcode
	* M. V. Wilkes, J. B. Stringer, [https://ur.booksc.eu/dl/53713365/d07ff7 Microprogramming and the design of the control circuits in an electronic digital computer], Proc. Cambridge Phil. Soc., Part 2, Volume 49, pp. 230-238, April, 1953 - Wilkes' full article on microcode <!-- https://gordonbell.azurewebsites.net/Computer_Structures__Readings_and_Examples/00000355.htm -->		* M. V. Wilkes, J. B. Stringer, [https://ur.booksc.eu/dl/53713365/d07ff7 Microprogramming and the design of the control circuits in an electronic digital computer], Proc. Cambridge Phil. Soc., Part 2, Volume 49, pp. 230-238, April, 1953 - Wilkes' full article on microcode <!-- https://gordonbell.azurewebsites.net/Computer_Structures__Readings_and_Examples/00000355.htm -->
	* S. G. Tucker, [https://dokumen.tips/documents/microprogram-control-for-system360.html ''Microprogram control for System/360''], IBM Systems Journal, Vol. 6, No.4, 1967 - provides a good general overview of the 'state of the art' at the time		* S. G. Tucker, [https://dokumen.tips/documents/microprogram-control-for-system360.html ''Microprogram control for System/360''], IBM Systems Journal, Vol. 6, No.4, 1967 - provides a good general overview of the 'state of the art' at the time

	==External links==		==External links==

Jnc: /* Further reading */ +Microprogram control for System/360

2024-01-22T12:25:41Z

Further reading: +Microprogram control for System/360

← Older revision		Revision as of 12:25, 22 January 2024
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	Once a microcoded CPU had been designed, it was relatively easy to expand the [[instruction set]] to include more complex instructions (something whose only per-unit repeated cost was larger microcode memories). For much of the lifetime of computers, the CPU was faster than the main memory; in such an environment, it made sense to make the instructions of the CPU as powerful (complicated) as possible, so a single instruction fetch (which is, of course, pure overhead in terms of the actual computation) does as much as possible. Such complex instructions are more easily implemented with a microcode approach, than with combinatorial logic.		Once a microcoded CPU had been designed, it was relatively easy to expand the [[instruction set]] to include more complex instructions (something whose only per-unit repeated cost was larger microcode memories). For much of the lifetime of computers, the CPU was faster than the main memory; in such an environment, it made sense to make the instructions of the CPU as powerful (complicated) as possible, so a single instruction fetch (which is, of course, pure overhead in terms of the actual computation) does as much as possible. Such complex instructions are more easily implemented with a microcode approach, than with combinatorial logic.

	==~~External links~~==		==Further reading==

	* M. V. Wilkes, [https://www.cs.princeton.edu/courses/archive/fall10/cos375/BestWay.pdf The Best Way to Design an Automatic Computing Machine], Report of Manchester University Computer Inaugural Conference, July 1951, pp. 16–18 - Wilkes' original note on microcode		* M. V. Wilkes, [https://www.cs.princeton.edu/courses/archive/fall10/cos375/BestWay.pdf The Best Way to Design an Automatic Computing Machine], Report of Manchester University Computer Inaugural Conference, July 1951, pp. 16–18 - Wilkes' original note on microcode
	* M. V. Wilkes, J. B. Stringer, [https://ur.booksc.eu/dl/53713365/d07ff7 Microprogramming and the design of the control circuits in an electronic digital computer], Proc. Cambridge Phil. Soc., Part 2, Volume 49, pp. 230-238, April, 1953 - Wilkes' full article on microcode <!-- https://gordonbell.azurewebsites.net/Computer_Structures__Readings_and_Examples/00000355.htm -->		* M. V. Wilkes, J. B. Stringer, [https://ur.booksc.eu/dl/53713365/d07ff7 Microprogramming and the design of the control circuits in an electronic digital computer], Proc. Cambridge Phil. Soc., Part 2, Volume 49, pp. 230-238, April, 1953 - Wilkes' full article on microcode <!-- https://gordonbell.azurewebsites.net/Computer_Structures__Readings_and_Examples/00000355.htm -->
			* S. G. Tucker, [https://dokumen.tips/documents/microprogram-control-for-system360.html ''Microprogram control for System/360''], IBM Systems Journal, Vol. 6, No.4, 1967 - provides a good general overview of the 'state of the art' at the time

			==External links==

	* [http://www.edwardbosworth.com/CPSC5155/PH4/Ch04/Ch04_Microprogramming.pdf Control Units: Hardwired & Microprogrammed]		* [http://www.edwardbosworth.com/CPSC5155/PH4/Ch04/Ch04_Microprogramming.pdf Control Units: Hardwired & Microprogrammed]
	* [https://bitsavers.org/pdf/ibm/370/GF20-0385-0_An_Introduction_to_Microprogramming_Dec71.pdf An Introduction to Microprogramming]		* [https://bitsavers.org/pdf/ibm/370/GF20-0385-0_An_Introduction_to_Microprogramming_Dec71.pdf An Introduction to Microprogramming]

	[[Category: Hardware Basics]]		[[Category: Hardware Basics]]

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2024-01-21T23:18:29Z

External links: typo

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	* M. V. Wilkes, J. B. Stringer, [https://ur.booksc.eu/dl/53713365/d07ff7 Microprogramming and the design of the control circuits in an electronic digital computer], Proc. Cambridge Phil. Soc., Part 2, Volume 49, pp. 230-238, April, 1953 - Wilkes' full article on microcode <!-- https://gordonbell.azurewebsites.net/Computer_Structures__Readings_and_Examples/00000355.htm -->		* M. V. Wilkes, J. B. Stringer, [https://ur.booksc.eu/dl/53713365/d07ff7 Microprogramming and the design of the control circuits in an electronic digital computer], Proc. Cambridge Phil. Soc., Part 2, Volume 49, pp. 230-238, April, 1953 - Wilkes' full article on microcode <!-- https://gordonbell.azurewebsites.net/Computer_Structures__Readings_and_Examples/00000355.htm -->
	* [http://www.edwardbosworth.com/CPSC5155/PH4/Ch04/Ch04_Microprogramming.pdf Control Units: Hardwired & Microprogrammed]		* [http://www.edwardbosworth.com/CPSC5155/PH4/Ch04/Ch04_Microprogramming.pdf Control Units: Hardwired & Microprogrammed]
	* [https://bitsavers.org/pdf/ibm/370/GF20-0385-0_An_Introduction_to_Microprogramming_Dec71.pdf An Introduction to		* [https://bitsavers.org/pdf/ibm/370/GF20-0385-0_An_Introduction_to_Microprogramming_Dec71.pdf An Introduction to Microprogramming]
	Microprogramming]

	[[Category: Hardware Basics]]		[[Category: Hardware Basics]]

Jnc: /* External links */ +An Introduction to Microprogramming

2024-01-21T23:18:03Z

External links: +An Introduction to Microprogramming

← Older revision		Revision as of 23:18, 21 January 2024
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	* M. V. Wilkes, J. B. Stringer, [https://ur.booksc.eu/dl/53713365/d07ff7 Microprogramming and the design of the control circuits in an electronic digital computer], Proc. Cambridge Phil. Soc., Part 2, Volume 49, pp. 230-238, April, 1953 - Wilkes' full article on microcode <!-- https://gordonbell.azurewebsites.net/Computer_Structures__Readings_and_Examples/00000355.htm -->		* M. V. Wilkes, J. B. Stringer, [https://ur.booksc.eu/dl/53713365/d07ff7 Microprogramming and the design of the control circuits in an electronic digital computer], Proc. Cambridge Phil. Soc., Part 2, Volume 49, pp. 230-238, April, 1953 - Wilkes' full article on microcode <!-- https://gordonbell.azurewebsites.net/Computer_Structures__Readings_and_Examples/00000355.htm -->
	* [http://www.edwardbosworth.com/CPSC5155/PH4/Ch04/Ch04_Microprogramming.pdf Control Units: Hardwired & Microprogrammed]		* [http://www.edwardbosworth.com/CPSC5155/PH4/Ch04/Ch04_Microprogramming.pdf Control Units: Hardwired & Microprogrammed]
			* [https://bitsavers.org/pdf/ibm/370/GF20-0385-0_An_Introduction_to_Microprogramming_Dec71.pdf An Introduction to
			Microprogramming]

	[[Category: Hardware Basics]]		[[Category: Hardware Basics]]

Jnc: /* Enhancements */ typo, clarify

2022-02-04T22:55:30Z

Enhancements: typo, clarify

← Older revision		Revision as of 22:55, 4 February 2022
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	==Enhancements==		==Enhancements==

	In 1951, [[Maurice Wilkes]] suggested more widespread use of that technique (to get rid of the design burden, and chance of errors, in using logic aggregations for control; he added [[conditional branch]]ing, to make microcode more flexible. Over time, more concepts from [[higher-level language]]s have been added to microcode, such as [[subroutine]]s.		In 1951, [[Maurice Wilkes]] suggested more widespread use of that technique (to get rid of the design burden, and chance of errors, in using un-structured logic aggregations for control); he added [[conditional branch]]ing, to make microcode more flexible. Over time, more concepts from [[higher-level language]]s have been added to microcode, such as [[subroutine]]s.

	Although early microcode was implemented in read-only memory (the Whirlwind ROM was a [[diode]] matrix), some later computers included high-speed [[Random Access Memory\|RAM]] for part of the control store, so that the instruction set could be dynamically extended to speed up applications.		Although early microcode was implemented in read-only memory (the Whirlwind ROM was a [[diode]] matrix), some later computers included high-speed [[Random Access Memory\|RAM]] for part of the control store, so that the instruction set could be dynamically extended to speed up applications.

Jnc: /* Enhancements */ link MW

2022-02-03T20:13:07Z

Enhancements: link MW

← Older revision		Revision as of 20:13, 3 February 2022
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	==Enhancements==		==Enhancements==

	In 1951, Maurice Wilkes suggested more widespread use of that technique (to get rid of the design burden, and chance of errors, in using logic aggregations for control; he added [[conditional branch]]ing, to make microcode more flexible. Over time, more concepts from [[higher-level language]]s have been added to microcode, such as [[subroutine]]s.		In 1951, [[Maurice Wilkes]] suggested more widespread use of that technique (to get rid of the design burden, and chance of errors, in using logic aggregations for control; he added [[conditional branch]]ing, to make microcode more flexible. Over time, more concepts from [[higher-level language]]s have been added to microcode, such as [[subroutine]]s.

	Although early microcode was implemented in read-only memory (the Whirlwind ROM was a [[diode]] matrix), some later computers included high-speed [[Random Access Memory\|RAM]] for part of the control store, so that the instruction set could be dynamically extended to speed up applications.		Although early microcode was implemented in read-only memory (the Whirlwind ROM was a [[diode]] matrix), some later computers included high-speed [[Random Access Memory\|RAM]] for part of the control store, so that the instruction set could be dynamically extended to speed up applications.

Jnc: Other uses, ext links to original papers

2022-02-03T17:25:49Z

Other uses, ext links to original papers

← Older revision		Revision as of 17:25, 3 February 2022
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	==Enhancements==		==Enhancements==

	In 1951, Maurice Wilkes suggested ~~adding~~ [[conditional branch]]ing, to make microcode more flexible. Over time, more concepts from [[higher-level language]]s have been added to microcode, such as [[subroutine]]s.		In 1951, Maurice Wilkes suggested more widespread use of that technique (to get rid of the design burden, and chance of errors, in using logic aggregations for control; he added [[conditional branch]]ing, to make microcode more flexible. Over time, more concepts from [[higher-level language]]s have been added to microcode, such as [[subroutine]]s.

	Although early microcode was implemented in read-only memory (the Whirlwind ROM was a diode matrix), some later computers included high-speed [[Random Access Memory\|RAM]] for part of the control store, so that the instruction set could be dynamically extended to speed up applications.		Although early microcode was implemented in read-only memory (the Whirlwind ROM was a [[diode]] matrix), some later computers included high-speed [[Random Access Memory\|RAM]] for part of the control store, so that the instruction set could be dynamically extended to speed up applications.

	Recently, microcode has been augmented with [[programmable logic array]]s, which are usually given the job of decoding instructions (something done more efficiently in combinatorial logic).		Recently, microcode has been augmented with [[programmable logic array]]s, which are usually given the job of decoding instructions (something done more efficiently in combinatorial logic).

	==Other advantages==		==Other advantages and uses==

			Once microprogramming came into more general use, a number of other advantages, and uses, appeared.

	With much of the control for a microprogrammed machine in the microprogram, bugs in the CPU could often be fixed by replacing the microprogram alone, without required hardware changes. (With the coming of the [[microprocessor]], where the microprogram is often in ROM in the chip, this is usually no longer possible.)		With much of the control for a microprogrammed machine in the microprogram, bugs in the CPU could often be fixed by replacing the microprogram alone, without required hardware changes. (With the coming of the [[microprocessor]], where the microprogram is often in ROM in the chip, this is usually no longer possible.)

	Microprogramming also made it possible to offer a range (in performance and cost terms) of machines which all had the same basic architecture, but different internal implementations; microcode could be used to make the different machines all support the same instruction set. This too is now not common, since it is not usual to offer multiple implementations with differing performance at the same time; however, it can be used to meet an analogous goal, making new generations of microprocessors implement the identical instruction set to previous ones.		Microprogramming also made it possible to offer a range (in performance and cost terms) of machines which all had the same basic architecture, but different internal implementations; microcode could be used to make the different machines all support the same instruction set. This was originally, and most influentially, displayed by the [[IBM System/360]] line of computers. This too is now not common, since it is not usual to offer multiple implementations with differing performance at the same time; however, it can be used to meet an analogous goal, making new generations of microprocessors implement the identical instruction set to previous ones.

			The System/360 also showed another use for microcode: System/360 computers included microcode to [[emulate]] earlier IBM computers (such as the [[IBM 1401]]); in an era when most programming was done in [[assembly language]], this allowed people to upgrade to a System/360 machine without discarding their investment in software for the earlier machine.

			Once a microcoded CPU had been designed, it was relatively easy to expand the [[instruction set]] to include more complex instructions (something whose only per-unit repeated cost was larger microcode memories). For much of the lifetime of computers, the CPU was faster than the main memory; in such an environment, it made sense to make the instructions of the CPU as powerful (complicated) as possible, so a single instruction fetch (which is, of course, pure overhead in terms of the actual computation) does as much as possible. Such complex instructions are more easily implemented with a microcode approach, than with combinatorial logic.

			==External links==

	~~For much~~ of the ~~lifetime~~ of ~~computers,~~ the ~~CPU was faster than the main memory;~~ in ~~such~~ an ~~environment~~, ~~it makes sense to make the instructions of the CPU as powerful (complicated) as possible~~, ~~so a single instruction fetch (which is~~, ~~of course~~, ~~pure overhead in terms of the actual computation) does as much as possible~~. ~~Such complex instructions are more easily implemented with a~~ microcode ~~approach, than with combinatorial logic~~.		* M. V. Wilkes, [https://www.cs.princeton.edu/courses/archive/fall10/cos375/BestWay.pdf The Best Way to Design an Automatic Computing Machine], Report of Manchester University Computer Inaugural Conference, July 1951, pp. 16–18 - Wilkes' original note on microcode
			* M. V. Wilkes, J. B. Stringer, [https://ur.booksc.eu/dl/53713365/d07ff7 Microprogramming and the design of the control circuits in an electronic digital computer], Proc. Cambridge Phil. Soc., Part 2, Volume 49, pp. 230-238, April, 1953 - Wilkes' full article on microcode <!-- https://gordonbell.azurewebsites.net/Computer_Structures__Readings_and_Examples/00000355.htm -->
			* [http://www.edwardbosworth.com/CPSC5155/PH4/Ch04/Ch04_Microprogramming.pdf Control Units: Hardwired & Microprogrammed]

	[[Category: Hardware ~~Basics‎‎~~]]		[[Category: Hardware Basics]]

Jnc: /* Other advantages */ typo

2020-12-12T23:33:18Z

Other advantages: typo

← Older revision		Revision as of 23:33, 12 December 2020
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	Microprogramming also made it possible to offer a range (in performance and cost terms) of machines which all had the same basic architecture, but different internal implementations; microcode could be used to make the different machines all support the same instruction set. This too is now not common, since it is not usual to offer multiple implementations with differing performance at the same time; however, it can be used to meet an analogous goal, making new generations of microprocessors implement the identical instruction set to previous ones.		Microprogramming also made it possible to offer a range (in performance and cost terms) of machines which all had the same basic architecture, but different internal implementations; microcode could be used to make the different machines all support the same instruction set. This too is now not common, since it is not usual to offer multiple implementations with differing performance at the same time; however, it can be used to meet an analogous goal, making new generations of microprocessors implement the identical instruction set to previous ones.

	For much of the lifetime of computers, the CPU was faster than the main memory; in such an environment, it makes sense to make the instructions of the CPU as ~~high-level~~ as possible, so a single instruction fetch (which is, of course, pure overhead in terms ot the actual computation) does as much as possible. Such complex instructions are more easily implemented with a microcode approach, than with combinatorial logic.		For much of the lifetime of computers, the CPU was faster than the main memory; in such an environment, it makes sense to make the instructions of the CPU as powerful (complicated) as possible, so a single instruction fetch (which is, of course, pure overhead in terms of the actual computation) does as much as possible. Such complex instructions are more easily implemented with a microcode approach, than with combinatorial logic.

	[[Category: Hardware Basics‎‎]]		[[Category: Hardware Basics‎‎]]

← Older revision		Revision as of 08:49, 16 October 2026
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	==Further reading==		==Further reading==

	* M. V. Wilkes, [https://www.cs.princeton.edu/courses/archive/fall10/cos375/BestWay.pdf The Best Way to Design an Automatic Computing Machine], Report of Manchester University Computer Inaugural Conference, July 1951, pp. 16–18 - Wilkes' original note on microcode		* M. V. Wilkes, [https://www.cs.princeton.edu/courses/archive/fall10/cos375/BestWay.pdf The Best Way to Design an Automatic Computing Machine], Report of Manchester University Computer Inaugural Conference, July 1951, pp. 16–18 - Wilkes' original note on microcode (re-printed in ''The Early British Computer Conferences'', MIT Press, Cambridge, 1989)
	* M. V. Wilkes, J. B. Stringer, [https://ur.booksc.eu/dl/53713365/d07ff7 Microprogramming and the design of the control circuits in an electronic digital computer], Proc. Cambridge Phil. Soc., Part 2, Volume 49, pp. 230-238, April, 1953 - Wilkes' full article on microcode <!-- https://gordonbell.azurewebsites.net/Computer_Structures__Readings_and_Examples/00000355.htm -->		* M. V. Wilkes, J. B. Stringer, [https://ur.booksc.eu/dl/53713365/d07ff7 Microprogramming and the design of the control circuits in an electronic digital computer], Proc. Cambridge Phil. Soc., Part 2, Volume 49, pp. 230-238, April, 1953 - Wilkes' full article on microcode <!-- https://gordonbell.azurewebsites.net/Computer_Structures__Readings_and_Examples/00000355.htm -->
	* S. G. Tucker, [https://dokumen.tips/documents/microprogram-control-for-system360.html ''Microprogram control for System/360''], IBM Systems Journal, Vol. 6, No.4, 1967 - provides a good general overview of the 'state of the art' at the time		* S. G. Tucker, [https://dokumen.tips/documents/microprogram-control-for-system360.html ''Microprogram control for System/360''], IBM Systems Journal, Vol. 6, No.4, 1967 - provides a good general overview of the 'state of the art' at the time