GAL: GAL Assembly Language - User Manual
1. Introduction
GAL is a small programming language for a small machine: one accumulator, twenty-two instructions, and a front panel with a code playground.
The web page is a working interpreter designed to run programs written in GAL. The instruction set is defined in section 5 below.
The current version of GAL is 1.0.
2. Writing a GAL Program
The format of instructions is:
[LABEL:] MNEMONIC [OPERAND]
Labels and mnemonics are not case sensitive, so HLT, hlt and Hlt are all the same thing. There should be one or more spaces between label and mnemonic, and between mnemonic and operand (if there is one).
Except for data (DAT) definitions, LABEL is optional. A label may comprise letters, digits and underscores but cannot begin with a numerical digit. Every label declaration ends with a colon, including on DAT lines.
MNEMONIC must be specified – the only exception is that a line may consist of just a label, in which case the label refers to the instruction following.
Depending on the mnemonic, OPERAND may be optional, and where it is required may be:
- the name of a data cell
- a code label
- an immediate constant.
An immediate constant must be a whole number (an integer) optionally preceded by a + or - sign. An operand that begins with a digit, a plus sign or a minus sign is treated as an immediate constant and must be a valid whole number; anything else is taken to be the name of a data cell or label.
For a data cell, a label is required, and it may or may not have an operand. If it has an operand, this must be an immediate constant and this sets the data cell's initial value. If no initial value is specified, it will be set to zero.
Anything following a semicolon is treated as a comment and is ignored.
Enter the program with each instruction on a separate line. Whilst it is not necessary, horizontally lining up the mnemonics and operands helps when reading the program. Blank lines can also aid readability. Section 5 gives a list of available instructions.
3. Numbers
The machine considers everything as a number, so letters and symbols are stored as numbers. The sample program text2numbers can be used to find the character code for any particular characters. Note that the character code for uppercase and lowercase letters is different.
Numbers are whole numbers, positive or negative, limited only by the engine itself; arithmetic that overflows is an error.
All arithmetic is done in the Accumulator. Data can be loaded into the accumulator from a data cell or read from the input stream, operated upon, stored in a data cell, or printed.
Printing is the program's responsibility; to move to a new line, use LOD 10 followed by PRC, and to add a space, use LOD 32 plus PRC.
Note that ADD and SUB may use positive or negative constants and this may cause some confusion at first: ADD -4 has the same result as SUB 4 – both subtract 4 from the value in the accumulator.
It might also be helpful to note that a digit may be read as a character and converted to a number simply by subtracting 48.
4. Running a Program
GAL has an area to enter a program. By default, it loads a simple demo program. You may delete the program (on Windows, click inside the box, press Ctrl+A and then Delete) and type your own program, or you can choose to run one of the sample programs. You could copy and paste a program prepared in, say, Notepad if you wish. The program box is resizable if you want to see more of the program at a time.
When running a program, if it expects any input, this must be entered in the Input stream before pressing Run.
If you want to see the values held in each data cell when the program stops, click the checkbox labelled dump data cells after running.
Any output, if there is any, will be shown in the Output box.
The panel lights show the lowest bits of each value – 16 bits for the accumulator, 8 for the program counter; the decimal values alongside are always exact. The program counter gives the number of the next instruction to run, counting from 0 and ignoring comments, labels and DAT lines. Error messages, on the other hand, quote the source line, which counts every line including comments and blanks – so the two numbers usually differ. The panel also shows whether or not the program has terminated correctly. If the Halt light is amber it means the program has terminated correctly; if the light is red then it means an error has occurred.
If GAL has found an error, this will be shown below the panel. The error message gives the line number of the instruction in error and a clear indication of what is wrong.
5. The Instruction Set
The following table is a full list of valid mnemonics:
| Mnemonic | Description |
|---|---|
| CLR | clear the accumulator |
| LOD | load the accumulator from a cell or with a constant |
| STO | store the accumulator into a data cell |
| ADD | add a cell or constant to the accumulator |
| SUB | subtract a cell or constant from the accumulator |
| INC | add 1 to the accumulator |
| DEC | subtract 1 from the accumulator |
| NEG | negate the value in the accumulator |
| MUL | multiply the accumulator by a cell or constant |
| DIV | divide the accumulator by a cell or constant |
| MOD | modulo divide the accumulator by a cell or constant |
| RDN | read a number into the accumulator (error if no number remains) |
| RDC | read the next character into the accumulator (0 when input ends) |
| PRN | output the accumulator as a number |
| PRC | output the accumulator as a character |
| JMP | jump to a code label |
| JEZ | jump to a code label if the accumulator is zero |
| JNZ | jump to a code label if the accumulator is not zero |
| JLZ | jump to a code label if the accumulator is less than zero |
| JGZ | jump to a code label if the accumulator is greater than zero |
| HLT | stop execution |
| NOP | do nothing |
| DAT | declare a data cell (label required), e.g. FOO: DAT 10 |
Last updated: 2026-10-29 13:52