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 to carry out computations

Registers have no datatypes

For example, add in MIPS assumes the data is in 2’s Complement (补码) and addu assumes the data is unsigned binary number.

The data stored in the register is interpreted according to the instruction that uses it.

Register Allocation

• Compiler associates Register with variables in the given program

Memory Register

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Memory Address Register

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

Memory Data Register

• A register temporarily stores data that is being transferred to or retrieved from memory

Common Registers

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Instruction Register

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

Program Counter

• Also known as Instruction Address Register, Instruction Pointer
• 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

Terminologies

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Register Width

• The number of Bit a Register can hold

Register File

• A collection of Register that CPU can use to 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