Compiler Concept

Transpiler:

A transpiler (short for source-to-source compiler) is a tool that transforms code from one representation to another, typically at the same or similar level of abstraction.

In general:

• It doesn’t compile down to machine code, like a traditional compiler.
• Instead, it rewrites code from one language or format into another that is structurally similar or semantically equivalent.

Understanding AUIPC in RISC-V

AUIPC stands for Add Upper Immediate to Program Counter. It’s a RISC-V instruction used to compute PC-relative addresses, often to support position-independent code or access global data.

🔧 AUIPC Syntax

auipc rd, imm

This computes:

rd = PC + (imm << 12)

The 20-bit immediate is shifted left by 12 bits (multiplying it by 4096), then added to the current program counter (PC). The result is stored in register rd.

🔁 Example Use of AUIPC

auipc t0, 0x1       # t0 = PC + 0x1_000 = PC + 4096
lw    t1, -400(t0)  # t1 = mem[t0 - 400]

This pattern helps access a 32-bit address even though lw only supports a 12-bit offset. This is essential in RISC-V because instructions are fixed at 32 bits, limiting how much data (like addresses) they can contain.

💬 Questions and Answers

Q: What is a word in RISC-V?

A word is 4 bytes (32 bits) of data. Instructions like lw (load word) and sw (store word) operate on 4-byte chunks.

Q: What is an immediate?

An immediate is a constant number encoded directly in an instruction. For example, in addi x5, x0, 10, the number 10 is an immediate.

Q: Why does AUIPC shift the immediate by 12?

AUIPC is meant to build 32-bit addresses using two instructions: it provides the upper 20 bits, and the second instruction (like lw or jalr) adds a lower 12-bit signed immediate.

Q: Why is the immediate only 12 bits in instructions like lw, sw, and jalr?

RISC-V instructions are fixed-size (32 bits), and the immediate field must share space with other parts like the opcode and register fields. A 12-bit signed immediate is a good compromise, offering a ±2048 byte range.

Q: What is the syntax of lw, sw, and jalr in RISC-V?

• lw rd, imm(rs1): Load 4 bytes from memory at rs1 + imm into rd.
• sw rs2, imm(rs1): Store 4 bytes from rs2 into memory at rs1 + imm.
• jalr rd, imm(rs1): Jump to rs1 + imm, and store return address in rd.

Q: What are x5 and x6 in RISC-V?

These are general-purpose registers. RISC-V has 32 registers named x0 to x31. Each also has a conventional name:

• x5 is t0 (temporary)
• x6 is t1 (temporary)

📚 Summary Table

Instruction	Operation	Immediate Range
lw	rd = *(rs1 + imm)	-2048 to +2047
sw	*(rs1 + imm) = rs2	-2048 to +2047
jalr	PC = rs1 + imm; rd = return address	-2048 to +2047

This overview should help you understand how AUIPC works and why it plays a key role in address computation in RISC-V.