CESIL

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Short description: Educational computer programming language

CESIL, or Computer Education in Schools Instruction Language,[1] is a programming language designed to introduce pupils in British secondary schools to elementary computer programming. It is a simple language containing a total of fourteen instructions.

Background

Computer Education in Schools (CES) was a project developed in the late 1960s and early 1970s by International Computers Limited (ICL).[2] CESIL was developed by ICL as part of the CES project, and introduced in 1974.[3] In those days, very few if any schools had computers, so pupils would write programs on coding sheets, which would then be transferred to punched cards or paper tape.[4] Typically, this would be sent to run on a mainframe computer, with the output from a line printer being returned later.[5]

Structure

Because CESIL was not designed as an interactive language, there is no facility to input data in real time. Instead, numeric data is included as a separate section at the end of the program.[6]

The fundamental principle of CESIL is the use of a single accumulator, which handles mathematical operations.[4] Numeric values are stored in variables, which in CESIL are referred to as store locations.[7] CESIL only works with integers, and results from DIVIDE operations are rounded if necessary.[8] There is no facility for data structures such as arrays, nor for string handling, though string constants can be output by means of the PRINT instruction.[4]

Jumps and loops can be conditional or non-conditional, and transfer operation of the program to a line with a specific label, which is identified in the first column of a coding sheet.[9] The instruction or operation is stated in the second column, and the operand in the third column.[10] On some coding sheets, comments and the text of the PRINT instruction would be written in a fourth column.[11]

Instructions

Instructions, or operations, are written in upper case and may have a single operand, which can be a store location, constant integer value or line label. Store locations and line labels are alphanumeric, up to six characters, and begin with a letter.[12] Numeric integer constants must be signed + or −, with zero being denoted as +0.[13][lower-alpha 1]

Input and output

  • IN – reads the next value from the data, and stores it in the accumulator.[4] The error message *** PROGRAM REQUIRES MORE DATA *** is printed if the program tries to read beyond the end of the data provided.[14]
  • OUT – prints the current value of the accumulator. No carriage return is printed.[15]
  • PRINT "text in quotes" – prints the given text. No carriage return is printed.[15]
  • LINE – prints a carriage return, thus starting a new line.[16]

Memory storage

  • LOAD location or LOAD constant – copies the value of the given location or constant to the accumulator.[17]
  • STORE location – copies the contents of the accumulator to the given location.[10]

Mathematical instructions

  • ADD location or ADD constant – adds the value of the given location or constant to the accumulator.[18]
  • SUBTRACT location or SUBTRACT constant – subtracts the value of the given location or constant from the accumulator.[19]
  • MULTIPLY location or MULTIPLY constant – multiplies the accumulator by the value of the given location or constant.[20]
  • DIVIDE location or DIVIDE constant – divides the accumulator by the value of the given location or constant.[21] The result is rounded down if the result is positive, and up if the result is negative.[8] A *** DIVISION BY ZERO *** error message is printed if the divisor is zero.[22]

In each case, the result of the operation is stored in the accumulator, replacing the previous value.

Program control

  • JUMP label – unconditionally transfers control to location labelled.[23]
  • JINEG label (Jump If NEGative) – transfers control to location labelled if the accumulator contains a negative value.[24]
  • JIZERO label (Jump If ZERO) – transfers control to location labelled if the accumulator contains zero.[9]
  • HALT – terminates the program.[15]

Other symbols

Three special symbols are used in CESIL at the beginnings of lines.

  • % is used to mark the end of the program and the start of data.[11]
  • * is used to mark the end of the data.[25]
  • ( is used at the start of a line to indicate a comment.[26][lower-alpha 2]

CESIL programming tools

An emulator for CESIL, designed to run on Windows and called Visual CESIL, is available as freeware.[27]

An interpreter for CESIL, designed to run on the Android platform and called Wyrm CESIL, is available as free to install.[28]

Example

The following totals the integers in the runtime data section until it encounters a negative value and prints the total. 

        LOAD    +0
LOOP    STORE   TOTAL
        IN
        JINEG   DONE
        ADD     TOTAL
        JUMP    LOOP

DONE    PRINT   "The total is: "
        LOAD    TOTAL
        OUT
        LINE
        HALT

%
1
2
3
-1

The output of the above program would be:

The total is: 6

See also

Bibliography

Monsoon, Colin C; Sewell, Ian R; Frances P, Vickers (1978). Computer Studies. Book 1. ICL Computer Education in Schools. ISBN 0-903885-17-4. 

Notes

  1. ↑ Visual CESIL does not require non-negative constants to be signed.
  2. ↑ Not supported in Visual CESIL. * is used instead.

References

  1. ↑ Computer Studies, page 71
  2. ↑ "ICL–CES: Computer Education in Schools". https://iclces.uk/. 
  3. ↑ Furber, Steve (January 2012). "Appendix J: Timeline of major developments in Computing in schools in England". Shut down or restart? The way forward for computing in UK schools. The Royal Society. p. 120. https://royalsociety.org/-/media/education/computing-in-schools/2012-01-12-computing-in-schools.pdf#page120. Retrieved 20 June 2021. 
  4. ↑ 4.0 4.1 4.2 4.3 Computer Studies, page 72
  5. ↑ "My First Program". https://iclces.uk/articles/first_cesil_program.html. 
  6. ↑ Computer Studies, page 82
  7. ↑ Computer Studies, page 76
  8. ↑ 8.0 8.1 Computer Studies, pages 93–94
  9. ↑ 9.0 9.1 Computer Studies, page 148
  10. ↑ 10.0 10.1 Computer Studies, page 77
  11. ↑ 11.0 11.1 Computer Studies, page 74
  12. ↑ Computer Studies, pages 96, 148
  13. ↑ Computer Studies, pages 97–99
  14. ↑ Computer Studies, page 201
  15. ↑ 15.0 15.1 15.2 Computer Studies, page 73
  16. ↑ Computer Studies, pages 199–200
  17. ↑ Computer Studies, pages 80, 97–98
  18. ↑ Computer Studies, pages 84, 97–98
  19. ↑ Computer Studies, pages 86, 97–98
  20. ↑ Computer Studies, pages 90, 97–98
  21. ↑ Computer Studies, pages 92, 97–98
  22. ↑ Computer Studies, page 145
  23. ↑ Computer Studies, page 198
  24. ↑ Computer Studies, page 154
  25. ↑ Computer Studies, page 83
  26. ↑ Computer Studies, page 164
  27. ↑ Andrew John Jacobs (20 June 2010). "Visual CESIL". http://www.obelisk.me.uk/cesil/. 
  28. ↑ Wyrm Software (13 October 2019). "Wyrm CESIL". https://play.google.com/store/apps/details?id=com.WyrmSoftware.WyrmCESIL. 



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