Register

Abstract

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• A group of Flip-flops
• Accessible via Instruction
• 2 main purposes
  1. Storing values during computations (like Memory Address & value)
  2. Configuring CPU

Common Registers

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Instruction Register

• Holds the actual Instruction that is currently being executed by the CPU

Memory Address Register

• Register that points to Memory Address that contains the data required by the current Instruction that is being executed

Program Counter

• For modern computers, it is the Register that indicates the Virtual Address of the next Instruction to be executed for the current Process (进程)

Stack Pointer

• A Register that holds the Memory Address of the top of the Stack Segment in the current execution context. Here is the Diagram

offset($sp)

Used to access a memory location relative to the current top of the stack.

; Decrement the stack pointer by 8 (allocate 8 bytes on the stack)
sub $sp, 8 
 
; Move the value at the memory location (sp + 4) into the eax register
mov 4($sp), %eax 

Frame Pointer

• Maintains a reference point within the current Stack Frame
• At the beginning of a function (prologue), the Frame Pointer is typically set to the current value of the Stack Pointer, establishing the base of the current stack frame
• Throughout the function: The FP remains relatively unchanged, offering a stable reference point

Control & Status Registers (CSRs)

• There are multiple such registers in the CPU, the main purpose is storing information about the current state of the processor or device
• CSRs are typically used to control various aspects of the processor’s operation, such as interrupt handling, memory management, and power management etc

MIPS Registers

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Terminologies

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Register Width

• The number of Bit a Register can hold

Register File

• A collection of registers that CPU can quickly store and retrieve Data

Memory Mapped Register

• Register that can be accessed via a Main Memory location
• This means that the register can be accessed using the same Instruction and addressing modes as memory.
• Often used to control hardware devices, such as IO Device (Like the UART in XV6) and memory controllers

XV6-RISCV Memory Mapped Registers

memlayout.h

// XV6-RISCV Kernel Codes, memlayout.h
 
// Physical memory layout
 
// qemu -machine virt is set up like this,
// based on qemu's hw/riscv/virt.c:
//
// 00001000 -- boot ROM, provided by qemu
// 02000000 -- CLINT
// 0C000000 -- PLIC
// 10000000 -- uart0 
// 10001000 -- virtio disk 
// 80000000 -- boot ROM jumps here in machine mode
//             -kernel loads the kernel here
// unused RAM after 80000000.
 
// the kernel uses physical memory thus:
// 80000000 -- entry.S, then kernel text and data
// end -- start of kernel page allocation area
// PHYSTOP -- end RAM used by the kernel
 
// qemu puts UART registers here in physical memory.
#define UART0 0x10000000L
#define UART0_IRQ 10

References

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• microcontroller - difference between memory address register(MAR) and program counter(PC)? - Stack Overflow