Instruction sets

Contents

• What are instruction sets?
• Useful resources
• Little Man Computer
• Assembly vs High-level vs Low-level
• RISC and CISC

What are instruction sets?

An instruction set is a limited set of text instructions that can be used to program a CPU.

Different processor architectures have their own instruction sets, and different Assembly code. The most common are ARM (used in phones and tablets), and AMD64 (64-bit desktop and laptop processors by AMD and Intel).

Assembly language is very efficient as you control exactly what the CPU is doing. For example, the FPS game .kkrieger is only 96 KB.

Useful resources

• Little Man Computer
• LMC Instruction Set

Little Man Computer

Try running this assembly code in the LMC!

        INP         /* Get input */
        STA A       /* Store input in accumulator */
LOOP    LDA A       /* Begin loop / load value from accumulator into register A */
        OUT
        SUB ONE
        STA A
        BRZ ENDTHIS
        BRA LOOP
ENDTHIS LDA A
        SUB A
        OUT
        HLT
A       DAT
ONE     DAT 1

When you press Assemble into RAM, you can see that the the Assembly code is assembled, then stored into RAM. Then hit Run.

The textual instructions are converted into numeric values. For example INP is turned into 901.

The blue blobs are data being returned from or sent to RAM. The red blobs are the registers being accessed.

Assembly vs High-level vs Low-level

Assemby language requires many more lines of code than high or low level programming languages.

In Assembly, a simple algorithm to sort a list of numbers can take up to 50 lines, compared to about 20 lines in C (high-level language), or just 5 in Python (high-level language).

High-level languages can fit multiple lines of assembly into a single line of their high-level language.

High-level languages work on many systems. It is up to the compiler to convert the source code into an executable binary for each architecture.

RISC and CISC

Reduced Instruction Set Computer

• Simpler hardware
• More complex to code
• Each instruction is only one CPU cycle
• Lower energy consumption
• Used in ARM processors
• Physically smaller (due to less complex circuitry), so less silicon needed to produce the processors
• Enters a sleep mode when not actively processing

Complex Instruction Set Computer

• More complex (and expensive) hardware
• Easier to code for
• Multiple instruction cycles per instruction
• Greater energy consumption
• Used in AMD and Intel processors
• Physically larger, so more silicon needed to produce the processors