MIPS Instruction Set Overview

MIPS Instruction Set Overview

• MIPS (Microprocessor without Interlocked Pipelined Stages) is a family of reduced instruction set computer (RISC) instruction set architectures (ISA) developed by MIPS Computer Systems, later MIPS Technologies, based in the United States.
• There are multiple versions of MIPS, including MIPS I, II, III, IV, and V, and five releases of MIPS32/64, for 32 and 64-bit implementations.
• Early MIPS architectures were 32-bit; 64-bit versions were developed later.

Instruction Set

• The instruction set is the repertoire of instructions a computer uses.
• Different computers have different instruction sets but often share common aspects.
• Early computers frequently had simple instruction sets, designed for simplified implementation.
• Many modern computers also use simple instruction sets.
• Understanding the hardware/software interface is key to understanding the language used in the hardware.
• Programs are compiled into an executable, composed of machine instructions. This executable must run on future machines. Different processors might read the same x86 instructions, but handle them differently.
• Java programs are converted into portable bytecode that is converted into machine instructions during execution (just-in-time compilation).
• Key design principals when defining the instruction set architecture (ISA) include keeping hardware simple so the chip runs fast, and keeping the instructions regular for easier decoding and scheduling.

MIPS Arithmetic

• Most MIPS instructions use 3 operands.
• Operand order is fixed (destination first).
• For example, in C code A = B + C, the MIPS code would be add $so, $s1, $s2. Here, $so, $s1, $s2. are associated with variables by the compiler.

Operands

• In C, "variables" are locations in computer memory.
• In hardware, accessing memory is relatively expensive.
• To improve efficiency, variables can be loaded into on-chip scratchpads (registers).
• Simplifying instructions: instructions (add, sub) should only operate on registers.
• The number of operands (variables) in a C program is often very large. The number of registers in the assembly language version is fixed.

Registers vs. Memory

• Arithmetic instructions must operate on registers.
• Only 32 registers are provided.
• The compiler associates variables with registers.
• What about programs with lots of variables? Registers must be used for most variables whenever possible.
• Registers are faster to access than memory.
• Operating on memory data requires load and store instructions, which adds to the execution time.
• The compiler should use registers as much as possible, only spilling to memory for less frequently accessed variables. Register optimization is important.

Register Allocation

• The compiler attempts to keep as many variables as possible in registers.
• Some variables cannot be allocated to registers: large arrays (too few registers), aliased variables (variables accessible through pointers in C), and dynamically allocated variables (heap and stack).

Memory Address

• The compiler manages data in memory. It knows where each variable is located.
• It provides the appropriate memory addresses for instructions (load-store).

Memory Organization

• Memory is viewed as a large, single-dimension array; an address is an index in this array.
• Byte addressing means successive addresses are one byte apart.
• Bytes are practical, but larger data items often use "words".
• For MIPS, a word is 32 bits (or 4 bytes).
• Registers hold 32 bits of data.
• For example, 232 bytes of memory space with byte addresses (0 to 2³² - 1), and 230 words with byte addresses (0,4,8,...,2³²-4).

Memory Layout: Alignment

• Words in memory are aligned. The least 2 significant bits of a word address are 00.

MIPS ISA Selected Instruction Set (example instructions)

• Detailed instruction set information, with examples and descriptions of various instructions (add, subtract, load, store).

Assembly Language vs. Machine Language

• Assembly language provides convenient symbolic representation (easier than writing down binary machine instructions).
• Examples: Destination first, and pseudo instructions.
• Machine language is the underlying reality. Destination is not necessarily first in the code.

References

• Specific publications about MIPS architecture(s) are listed.