RISC vs CISC stack

RISC vs. CISC: Understanding Stack Operations and Processor Architectures

Explore the fundamental differences between RISC and CISC processor architectures, focusing on how each handles stack operations, memory access, and instruction sets. This article delves into the design philosophies that shape modern computing.

The central processing unit (CPU) is the brain of any computer system, executing instructions and performing calculations. Over the decades, two dominant philosophies have emerged in CPU design: Reduced Instruction Set Computing (RISC) and Complex Instruction Set Computing (CISC). While both aim to execute programs efficiently, they approach the task with fundamentally different strategies, particularly concerning instruction sets, memory access, and how they interact with the program stack.

CISC Architecture: Complexity and Microcode

CISC processors, exemplified by Intel's x86 architecture, are characterized by a large and complex instruction set. A single CISC instruction can perform multiple low-level operations, such as memory access, arithmetic, and register manipulation, all in one go. This complexity often means that instructions vary in length and execution time. To manage this, CISC processors heavily rely on microcode – a layer of simpler, internal instructions that translate complex machine instructions into a sequence of basic operations the hardware can understand.

flowchart TD
    A[CISC Instruction] --> B{Microcode Unit}
    B --> C1[Load Operand 1]
    B --> C2[Load Operand 2]
    B --> C3[Perform Operation]
    B --> C4[Store Result]
    C1 --> D[Registers/Memory]
    C2 --> D
    C3 --> D
    C4 --> D
    D --> E[Next Instruction]
    style A fill:#f9f,stroke:#333,stroke-width:2px
    style B fill:#bbf,stroke:#333,stroke-width:2px
    style C1 fill:#ccf,stroke:#333,stroke-width:1px
    style C2 fill:#ccf,stroke:#333,stroke-width:1px
    style C3 fill:#ccf,stroke:#333,stroke-width:1px
    style C4 fill:#ccf,stroke:#333,stroke-width:1px
    style D fill:#ffc,stroke:#333,stroke-width:1px
    style E fill:#f9f,stroke:#333,stroke-width:2px

CISC Instruction Execution Flow via Microcode

When it comes to stack operations, CISC processors often include dedicated instructions for pushing and popping multiple registers or entire stack frames with a single instruction. For example, an instruction like PUSHALL might save all general-purpose registers onto the stack, or ENTER might set up a new stack frame for a function call, handling base pointer and stack pointer adjustments automatically. This reduces the number of instructions a programmer needs to write and the compiler needs to generate, but it makes the internal execution more complex and potentially slower per instruction cycle.

; Example CISC stack operations (x86 assembly)

PUSH EBP        ; Save base pointer
MOV EBP, ESP    ; Set new base pointer to current stack pointer
SUB ESP, 10h    ; Allocate 16 bytes for local variables

; ... function body ...

MOV ESP, EBP    ; Deallocate local variables
POP EBP         ; Restore old base pointer
RET             ; Return from function

Typical x86 (CISC) stack frame setup and teardown

RISC Architecture: Simplicity and Pipelining

RISC processors, such as ARM and MIPS, embrace a philosophy of simplicity. They feature a smaller, highly optimized instruction set where each instruction performs a very basic operation (e.g., load, store, add, branch). All instructions typically have a fixed length and execute in a single clock cycle. This design choice simplifies the hardware, allowing for more efficient pipelining – a technique where multiple instructions are processed in different stages simultaneously, much like an assembly line.

flowchart LR
    A[Fetch] --> B[Decode] --> C[Execute] --> D[Memory Access] --> E[Write Back]
    subgraph Instruction Pipeline
        A -- Instruction 1 --> B
        B -- Instruction 2 --> C
        C -- Instruction 3 --> D
        D -- Instruction 4 --> E
    end
    style A fill:#f9f,stroke:#333,stroke-width:2px
    style B fill:#bbf,stroke:#333,stroke-width:2px
    style C fill:#ccf,stroke:#333,stroke-width:2px
    style D fill:#ffc,stroke:#333,stroke-width:2px
    style E fill:#afa,stroke:#333,stroke-width:2px

RISC Processor Pipeline Stages

In RISC, stack operations are typically broken down into multiple, simpler instructions. For instance, pushing multiple registers onto the stack would involve a sequence of individual STORE instructions, each saving one register to a specific memory address, followed by an adjustment to the stack pointer. Similarly, popping would involve multiple LOAD instructions. While this means more instructions are executed for a given task, the simplicity and fixed execution time of each instruction allow for higher clock speeds and more efficient pipelining, often leading to faster overall execution.

; Example RISC stack operations (ARM assembly)

SUB SP, SP, #16     ; Allocate 16 bytes for local variables
STR R0, [SP, #0]    ; Save R0 to stack
STR R1, [SP, #4]    ; Save R1 to stack

; ... function body ...

LDR R1, [SP, #4]    ; Restore R1 from stack
LDR R0, [SP, #0]    ; Restore R0 from stack
ADD SP, SP, #16     ; Deallocate local variables
BX LR               ; Return from function

Typical ARM (RISC) stack frame setup and teardown

Impact on Compiler Design and Performance

The choice between RISC and CISC significantly impacts compiler design. For CISC, compilers can generate fewer, more powerful instructions, but they must contend with variable instruction lengths and complex addressing modes. For RISC, compilers generate more instructions, but these instructions are simpler and more predictable, making optimization techniques like instruction scheduling and pipelining easier to implement effectively. Ultimately, the goal for both architectures is to achieve high performance, but they take different paths to get there.