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1. Registers

Registers are tiny, very fast storage locations inside the CPU.

RV32I has 32 integer registers:

• x0 to x31

Each register is 32 bits wide.

You can think of registers as named boxes that hold values the CPU is actively using.

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2. Register Diagram

+---------------------------------------------+
| Register File                               |
+---------------------------------------------+
| x0  | x1  | x2  | x3  | ... | x30 | x31    |
+---------------------------------------------+

Each box stores one 32-bit value.

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3. Important Rule: x0 is Always Zero

x0 is special.

• Reading x0 always gives 0
• Writing to x0 has no effect

Examples:

addi x5, x0, 9     # x5 = 9
addi x0, x0, 7     # ignored, x0 stays 0

This makes x0 extremely useful.

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4. ABI Names

Registers also have ABI names used by programmers and compilers.

Common ones:

x0 = zero constant zero
x1 = ra return address
x2 = sp stack pointer
x5 = t0 temporary
x6 = t1 temporary
x7 = t2 temporary
x8 = s0/fp saved register / frame pointer
x9 = s1 saved register
x10 = a0 function argument / return value
x11 = a1 function argument / return value
x12 = a2
x13 = a3
x14 = a4
x15 = a5
x16 = a6
x17 = a7

You can write either x10 or a0 depending on style.

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5. Register Roles

Very simplified categories:

• zero register:
  x0
• temporaries:
  t0-t6
  used for temporary calculations
• saved registers:
  s0-s11
  preserved across function calls
• argument / return registers:
  a0-a7
  used to pass inputs and outputs
• special:
  ra, sp

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6. CPU Datapath Idea

A simplified view:

The CPU reads source registers, performs an operation, then may write the result back to a destination register.

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7. Example: Using Registers

Code:

addi x5, x0, 3
addi x6, x0, 7
add  x7, x5, x6

Meaning:

• x5 = 3
• x6 = 7
• x7 = 10

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8. Step-by-Step Execution Trace

Initial:
x0 = 0
x5 = 0
x6 = 0
x7 = 0

Step 1
Instruction = addi x5, x0, 3
Read:

• x0 = 0
  Compute:
• 0 + 3 = 3
  Write:
• x5 = 3

Step 2
Instruction = addi x6, x0, 7
Read:

• x0 = 0
  Compute:
• 0 + 7 = 7
  Write:
• x6 = 7

Step 3
Instruction = add x7, x5, x6
Read:

• x5 = 3
• x6 = 7
  Compute:
• 3 + 7 = 10
  Write:
• x7 = 10

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9. Register Data Dependency

Notice the third instruction depends on earlier results.

addi x5, x0, 3
addi x6, x0, 7
add  x7, x5, x6

The add instruction needs the values produced by the first two instructions.

This idea becomes important later in pipeline hazards.

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10. Signed Integers

RV32I registers hold 32-bit patterns.
Those patterns may be interpreted as:

• signed integers
• unsigned integers
• memory addresses
• bit patterns

For signed values, two’s complement is used.

Examples:

• 0x00000005 = 5
• 0xFFFFFFFF = -1
• 0xFFFFFFFE = -2

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11. Common Beginner Mistakes

Mistake 1:
Thinking x0 can store a value.
It cannot.

Mistake 2:
Confusing register names and memory addresses.
Registers are inside the CPU; memory is outside the register file.

Mistake 3:
Assuming all registers are automatically preserved across function calls.
Only some are saved by convention.

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12. Exercises

1. How many integer registers are in RV32I?
2. What value is always read from x0?
3. What happens if you execute:
   addi x0, x0, 12
4. Which register is the stack pointer?
5. Which register usually stores a return address?
6. What is the value of x7 after:
   addi x5, x0, 8
   addi x6, x0, 2
   sub x7, x5, x6
7. Write a 2-instruction sequence that puts 20 into x9 using x0.
8. Why are registers faster than memory?

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13. Exercise Answers

1. 32
2. 0
3. Nothing changes; x0 stays 0.
4. x2, also called sp
5. x1, also called ra
6. 6
7. Example:
   addi x9, x0, 10
   addi x9, x9, 10
8. Because registers are built directly into the CPU and are designed for very fast access.