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.
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]
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, 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][a]
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]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]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.
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]Three special symbols are used in CESIL at the beginnings of lines.
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]
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