Assembly Language Instruction Set - A-Levels Computers

ARMlite Assembly Language Instruction Set

In the interest of giving learners real-life experiences, we have opted to reference ARMlite, which is a cut-down version of the assembly language instruction set used for programming the ARM chips that are found in a multitude of different devices, including Raspberry Pi computers. Teachers may choose to teach this in a purely theoretical way.

This list of Assembler instructions is a subset of the instruction set for the ARMlite simulator written by Peter Higginson.

ARMlite simulates a simple computer built around a cut-down version of a 32-bit Arm processor. It has 13 general-purpose registers (R0–R12) and three special-purpose registers. The first operand for all data processing operations must be a register. The second operand can be either an immediate value (#x) or a register (Rn).

Immediate values may be specified in decimal, hex or binary format, preceded by a #.

Comments are prefixed by a semicolon ;. They may be added after an instruction or on a line on their own.

Labels are used to specify the location of instructions for branching and to specify locations for loading and storing data. Labels must start with a letter and may be made up of upper- and lower-case letters and numbers.

ARMlite instruction reference

Instruction	Description
Arithmetic and logical operations
`ADD Rd, Rn, <operand>`	Add the value in `<operand>` to the value in register `Rn` and store the result in register `Rd`.
`SUB Rd, Rn, <operand>`	Subtract the value in `<operand>` from the value in register `Rn` and store the result in register `Rd`.
`AND Rd, Rn, <operand>`	Perform a bitwise logical AND between the values in register `Rn` and `<operand>`, and store the result in register `Rd`.
`ORR Rd, Rn, <operand>`	Perform a bitwise logical OR between the values in register `Rn` and `<operand>`, and store the result in register `Rd`.
`EOR Rd, Rn, <operand>`	Perform a bitwise logical XOR between the values in register `Rn` and `<operand>`, and store the result in register `Rd`.
`MVN Rd, <operand>`	Perform a bitwise logical NOT to the value in `<operand>`, and store the result in register `Rd`.
`LSL Rd, Rn, <operand>`	Logically shift the value stored in register `Rn` to the left by the number of bits specified by `<operand>` and store the result in register `Rd`.
`LSR Rd, Rn, <operand>`	Logically shift the value stored in register `Rn` to the right by the number of bits specified by `<operand>` and store the result in register `Rd`.
Comparison and branch instructions
`CMP Rn, <operand>`	Compare the value in `<operand>` with the value stored in register `Rn` and set status bits based on the outcome.
`B <label>`	Unconditionally branch to the address at position `<label>`.
`B<condition> <label>`	Conditionally branch to the instruction at position `<label>` when the last `CMP` instruction result matches the specified `<condition>`.
Data movement and other instructions
`MOV Rd, <operand>`	Move the value in `<operand>` into register `Rd`.
`LDR Rd, <address>`	Load the contents of the memory location `<address>` into register `Rd`.
`STR Rd, <address>`	Store the contents of `Rd` to the memory location `<address>`.
`HALT`	Halt program execution.
Syntax notes
`<operand>`	References either `#x` decimal value or `Rn` register `n`.
`<condition>`	`EQ` equal to, `NE` not equal to, `GT` greater than, `LT` less than.
`<label>:`	A label definition is formed by an identifier followed by a colon.
`Rd, Rn`	General-purpose registers `R0` to `R12`.