CESIL

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 1971.^[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 principal 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 structured data 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

Instuctions, 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 are signed + or -, with zero being denoted as +0.^[13][a]

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 value - place the immediate value or the contents of the variable named in the accumulator.
• Store variable - place the contents of the accumulator in the variable.

Mathematical instructions

• Add value - add the variable or immediate integer value to the accumulator.
• Subtract value - subtract the variable or immediate integer from the accumulator.
• Multiply value - place the product of the accumulator and the variable or immediate integer in the accumulator.
• Divide value - place the contents of the accumulator divided by the value in the accumulator.

Program control

• Jump label - transfer control to location labelled.
• Jineg label - transfer control to location labelled if the accumulator contains a negative value.
• Jizero label - transfer control to location labelled if the accumulator contains zero.
• Halt - return control to console.

Examples

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

[Output of the above program running...] 
        The total is:  6

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

References

External links

• http://www.obelisk.me.uk/cesil/