RISC (Reduced Instruction Set Computer) machine code is the lowest level of all programming languages. It is a computer-dependent design to control a specific machine architecture. The machine instructions manipulate the binary values stored in hardware registers through a central logical processing unit and addressable input and output logic. RISC architectures offer a small number of basic instructions executed quickly and efficiently that optimize performance through simplification to reduce the complexity of each functional step in any program.
RISC machine code is implemented in binary code, a sequence of 0s and 1s that defines the repertoire of individual comments. Each instruction performs simple binary tasks on data stored in hardware registers or shared memory systems. The instructions share the registers and the memory as a static linear address space, using a binary value to represent the various locations storing individual instruction parameters. Each instruction includes the input parameters, the command function to use, and the location for any result.
Conventionally the instructions are fixed-length binary words matching the common word size of the shared, centralized memory unit. This allows for efficient storage, simple encoding rules, and easy execution of overlapping instructions using a high-performance clock cycle that reads the next instruction, performs the instruction using the input parameters, and then saves the result before proceeding to the next instruction. The overlapping is limited by conditional test instruction that can change the instruction fetch address until some result is known.
The format of the binary machine code varies depending on the computer hardware, the word size, the number of programmable registers, and the statically shared address space. Still, common elements exist, as first proposed by Alan Turing. These include an opcode (operation code) to specify the command and one or two operands that store the data referenced by the instruction. These operands can be program-controlled registers, static memory addresses, or immediate values embedded in the instruction format.
For example, in this sixteen-bit instruction 1010100010000010, the opcode is the first four bits (1010), and the remaining bits (100010000010) represent the register (100) as the primary operand and other information defined by the instruction. Machine code is not directly written by a programmer but is generated by compilers or assemblers, reliably translating understandable human statements into machine code.
RISC stands for Reduced Instruction Set Computer; these computers emphasize simplicity and efficiency. The machine code is encoded in fixed-length proprietary binary formats, usually 32 bits in modern RISC computers. The instructions are fetched sequentially from memory and then executed individually by the computer, pipelined whenever possible to improve speed.
RISC machine code instructions are categorized by type based on functionality. The standard function types are:
- Load and Store Instructions that transfer data between the memory and the machine registers. Load instructions fetch data from memory and store it in a selected register, while store instructions save data from a selected register into a static binary location in shared memory. The operands are shared by each instruction so that results are also shared across any program or any other program that shares the same static address space.
- Arithmetic and Logical Instructions perform basic arithmetic and logical operations on data stored in registers. Examples include addition, subtraction, multiplication, division, and bitwise AND/OR/XOR operations.
- Transfer Instructions that control the flow of execution within and between programs. They include unconditional and conditional branch instructions, which change the shared memory address in the program counter to jump to a different location in shared memory based on a specified test result.
- Data Processing Instructions that perform operations on a specified register to shift, rotate, test, or otherwise manipulate binary data bits within a register.
- Special Purpose Instructions handle privileged hardware operations reserved for the central operating system or critical device drivers. They include interrupts, exceptions, virtual memory faults, and system service calls. These instructions allow interaction with the underlying operating system or handle exceptional conditions during program execution.
The specific machine code format and instructions like MIPS, ARM, or RISC-V all vary. Each computer defines a branded set of instructions with a unique encoding scheme. However, the general principles of shared static addressing, machine simplicity, instruction types, and efficient execution remain common to all RISC architectures. The nest result is they all supper from undetected malware and program bugs that deliberately or accidentally change a critical value in a shared location.
Furthermore, many individual programs shared the same linear, static memory space. Finally, to hide the Special Purpose Instructions. a superuser mode is required to limit access to any and all dangerous machine instructions. Here is an overview of the machine code formats for MIPS, ARM, and RISC-V architectures:
MIPS (Microprocessor without Interlocked Pipeline Stages): MIPS is a popular RISC architecture developed by MIPS Technologies. It follows a fixed-length instruction format with three common instruction types: R-type, I-type, and J-type.
- R-type Instructions (Register-type Instructions):
I-type Instructions (Immediate-type Instructions):
- | opcode 6 bits | rs 5 bits | rt 5 bits | rd 5 bits | shamt 5 bits | funct 6 bits |
J-type Instructions (Jump-type Instructions):
- |opcode 6 bits | rs 5 bits | rt 5 bits | immediate16 bits |
- | opcode 6 bits | address 26 bits |
ARM (Advanced RISC Machine): ARM is a widely used RISC architecture developed by ARM Holdings. It has a flexible instruction format called the ARM instruction set architecture (ISA) and is known for its variable-length instructions.
- 32-bit ARM Instruction:
16-bit Thumb Instruction:
- | cond 4 bits | opcode 4 bits | Rn 8 bits | Rd 8 bits | shamt 8 bits |
- | opcode 5 bits | Rn 3 bits | Rd 2 bits | offset 6 bits |
RISC-V: RISC-V is an open-source RISC architecture developed at the University of California, Berkeley. It has a modular design and supports various instruction formats.
- R-type Instructions (Register-type Instructions):
I-type Instructions (Immediate-type Instructions):
- |opcode 7 bits |rd 5 bits |funct3 3 bits |rs1 5 bits |rs2 5 bits | funct7 7 bits |
S-type Instructions (Store-type Instructions):
- |opcode 7 bits |rd 5 bits |funct3 3 bits |rs1 5 bits |imm[11:0] 12 bits |
B-type Instructions (Branch-type Instructions):
- |opcode 7 bits |imm[4:0] 5 bits |funct3 3 bits |rs1 5 bits |rs2 7 bits |imm[11:5] 5 bits |funct7 7 bits |
- | opcode 7 bits | imm 1 bit | imm[10:5] 6 bits | funct3 3 bits | rs1 5 bits | rs2 5 bits | imm[4:1,11] 5 bits | imm 7 bits |
Please note that the bit fields described above are general representations, and the actual format of instructions may vary depending on the specific instruction and architecture implementation within each family. Here is more details explanation of the machine code formats for MIPS, ARM, and RISC-V architectures:
MIPS (Microprocessor without Interlocked Pipeline Stages): R-type Instructions: R-type instructions in MIPS are used for register-based operations, such as arithmetic and logical operations. The format consists of six fields: opcode (6 bits), rs (5 bits), rt (5 bits), rd (5 bits), shamt (5 bits), and funct (6 bits). The opcode field indicates the type of instruction, the rs, rt, and rd fields specify the source and destination registers, the shamt field represents a shift amount for shift instructions, and the funct field specifies the specific operation to be performed.
I-type Instructions: I-type instructions in MIPS are used for immediate-based operations, such as loading data into registers or performing arithmetic operations on immediate values. The format consists of four fields: opcode (6 bits), rs (5 bits), rt (5 bits), and immediate (16 bits). The opcode field indicates the type of instruction, the rs field represents the source register, the rt field represents the destination register, and the immediate field contains the immediate value or memory offset.
J-type Instructions: J-type instructions in MIPS are used for jump instructions, which are used for altering the program flow by specifying a new address to jump to. The format consists of two fields: opcode (6 bits) and address (26 bits). The opcode field indicates the jump instruction, and the address field specifies the target address to jump to.
ARM (Advanced RISC Machine):
32-bit ARM Instruction: ARM instructions have a fixed 32-bit length. The format consists of five fields: cond (4 bits), opcode (4 bits), Rn (8 bits), Rd (8 bits), and shamt (8 bits). The cond field represents the condition under which the instruction is executed, the opcode field indicates the type of instruction, the Rn field specifies the first operand register, the Rd field specifies the destination register, and the shamt field represents a shift amount for shift instructions.
16-bit Thumb Instruction: ARM's Thumb instruction set supports 16-bit instructions, which are more compact. The format consists of four fields: opcode (5 bits), Rn (3 bits), Rd (2 bits), and offset (6 bits). The opcode field indicates the instruction type, the Rn and Rd fields specify the source and destination registers, and the offset field represents an immediate value or an offset.
R-type Instructions: R-type instructions in RISC-V are used for register-based operations. The format consists of seven fields: opcode (7 bits), rd (5 bits), funct3 (3 bits), rs1 (5 bits), rs2 (5 bits), and funct7 (7 bits). The opcode field indicates the instruction type, the rd field represents the destination register, the funct3 field specifies the operation category, the rs1 and rs2 fields represent the source registers, and the funct7 field further defines the operation.
I-type Instructions: I-type instructions in RISC-V are used for immediate-based operations. The format consists of five fields: opcode (7 bits), rd (5 bits), funct3 (3 bits), rs1 (5 bits), and imm[11:0] (12 bits). The opcode field indicates the instruction type, the rd field represents the destination register, the funct3 field specifies the operation category, the rs1 field represents the source register, and the imm[11:0] field contains the immediate value.
S-type Instructions: S-type instructions in RISC-V are used for storing data into memory. The format consists of seven fields
In the context of machine code formats, "rs1" refers to the field that represents the source register 1. It is an abbreviation for "register source 1." Similarly, "rd" represents the destination register, and it stands for "register destination."
In RISC architectures, instructions often involve operations on registers. The source registers hold the input values for an operation, and the destination register receives the result. The register names themselves (e.g., rs1, rd) are typically numeric identifiers associated with specific registers in the processor's register file.
For example, in the RISC-V instruction format:
| 7 bits | 5 bits | 3 bits | 5 bits | 5 bits | 7 bits |
| opcode | rd | funct3 | rs1 | rs2 | funct7 |
Here, "rs1" refers to the field that holds the identifier of the source register 1, while "rd" represents the field that holds the identifier of the destination register. The actual register names can vary depending on the architecture and its register naming convention.
In RISC architectures, such as RISC-V, the instruction formats often include additional fields beyond just the opcode, source registers, and destination registers. These additional fields serve specific purposes and provide more information about the instruction. Let's take a closer look at some of these fields:
funct3: The funct3 field is a 3-bit field that helps further specify the operation or operation category of the instruction. It works in conjunction with the opcode to determine the exact operation to be performed. Different values in the funct3 field correspond to different categories or variants of operations.
funct7: The funct7 field is a 7-bit field that, similar to funct3, provides additional information about the instruction's operation. It complements the funct3 field and, along with the opcode, helps specify the exact operation or variation.
imm[11:0]: The imm field represents an immediate value or a constant embedded in the instruction itself. The number of bits allocated for the immediate value varies depending on the architecture and instruction format. It allows instructions to operate on immediate values without needing to load them from memory or registers.
Other fields: The remaining fields, such as opcode, rd, rs1, and rs2, have been previously discussed. The opcode field indicates the type of instruction, rd represents the destination register, rs1 and rs2 represent the source registers, and they collectively specify the particular operation to be executed.
It's important to note that the specific bit lengths and meanings of these fields can vary across different RISC architectures. The examples provided earlier (RISC-V, MIPS, and ARM) showcase common conventions, but the actual formats may differ based on the specific implementation and design choices within each architecture family.