[CA] Lecture #07

Instruction

: The words of a computer's language.

- Instruction Set: vocabulary

• Different computers have different instruction sets.
• Early computers had very simple instruction sets.
• Many modern computers also have simple instruction sets.
• We will study RISC-V instruction set in this course.

 

The RISC-V Instruction Set

 

Stored-Program Concept

 

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Arithmetic Operations

: Add and subtract, three operands.

add  a, b, c   // a = b + c

▶ Design Principle 1: Simplicity favors regularity.

 

Arithmetic Example

f = (g + h) - (i + j);

add  t0, g, h
add  t1, i, j
sub  f, t0, t1

 

RISC-V Assembly Language

 

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Register Operands

• Arithmetic instructions use register operands.
• RISC-V has a 32 x 32-bit register file.
  - word: 32-bit (4-byte)
  - 32 x 32-bit general purpose registers x0 to x32.

▶ Design Principle 2: Smaller is faster.

 

RISC-V Registers

RISC-V Registers

 

Register Operand Example

f = (g + h) - (i + j);

add  x5, x20, x21   # x5: temporary variable
add  x6, x22, x23
sub  x19, x5, x6

 

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Memory Operands

• Main memory is used for composite data.
• To apply arithmetic operations, 
  - Load values from memory into registers.
  - Store result from register to memory.
• Memory is byte addressed.
  --> Each address identifies an 8-bit.
• RISC-V is Little Endian.
  - Little Endian: LSB at least address of a word.
  - Big Endian: MSB at least address of a word. (MIPS: Big Endian)
• RISC-V does not require words to be aligned in memory.

Little Endian vs. Big Endian

 

Memory Address

Memory Address

• Word --> 32 bits.
  - Byte --> 8 bits.

--> Since 8-bit bytes are useful in many programs, most architectures address individual bytes. (8-bit 즉, 1 byte 단위로 address에 접근한다.)

 

Load & Store Word

lw, sw

• Load word (lw): The data transfer instructions that copies data from memory to register.
• Store word (sw): The data transfer instructions that copies data from a register to memory.

 

Memory Operand Example

A[20] = h + A[8];

A[ ]

 

Register vs. Memory

• Registers are faster to access than memory.
• Operating on memory data requires loads and stores.
• Compiler must use registers for variables as much as possible.

 

Constant or Immediate Operand

• Many times, a program will use a constant in an operation.

addi  x22, x22, 4   # x22 = x22 + 4

 

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Summary: RISC-V Operands

• 0~2^32 bytes --> 2^30 words
  - 4 bytes = 1 word

 

Summary: RISC-V Assembly Language

 

The Constant Zero

• RISC-V register x0($zero) is the constant 0.
  --> Useful for common operations.

# Move between registers

add  x2, x1, x0   # x2 = x1 + 0

 

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Unsigned Numbers

Unsigned Numbers

 

2s-Complement Signed Integers

2s-Complement Signed Integers

▶ Bit 31 is sign bit.

• 1: -
• 0: +