RISC-V

Registers §

• In a 32-bit system, we may have 32 integer registers and 32 floating-point register.

Integer registers §

| Register Name | ABI | Name | Description | Saved by Calle- | |---|---|---|---| | x0 | zero | Always Zero | | | x1 | ra | Return address | R | | x2 | sp | Stack Pointer | E | | x3 | gp | Global pointer | | | x4 | tp | Thread pointer | | | x5 | t0 | Temporary / Alternate Return Address | R | | x6 - x7 | t1 - t2 | Temporary | R | | x8 | s0 / fp | Saved register / Frame Pointer | E | | x9 | s1 | Saved Register | E | | x10 - x11 | a0 - a1 | Function argument / return value | R | | x12 - x17 | a2 - a7 | Function argument | R | | x18 - x27 | s2 - s11 | Saved Register | E | | x28 - x31 | t3 - t6 | Temporary | R |

• Always Zero is Hardwired to always be zero. Any value moved to this register is discarded and zero outputted in its place.

Floating point registers §

Register Name	ABI Name	Description	Owner
f0 - f7	ft0-7	Temporaries	R
f8 - f9	fs0-1	Saved Registers	E
f10 - f11	fa0-1	Arguments / Return values	R
f12 - f17	fa2-7	Arguments	R
f18 - f27	fs2-11	Saved Registers	E
f28 - f31	ft8-11	Temporaries	R

• Note: We have multiple registers for return values in ode to support returning 64-bit values.
• We reserve register a7 typically for I/O (i.e., how to render output).

Variables §

• All variables are declared in .data

Directives	Description
.byte	8 bit integer
.half	16-bit integer
.word	32-bit integer
.dword	64-bit integer
.float	32-bit single precision floating point
.double	64-bit double precision floating point
.asciz	string with null terminator

• All memory locations are viewed as 1 byte with Little Endian ordering and 32-bit words for a 32-bit system.
• Memory is used for composite data such as arrays and structures

Addressing Modes §

• Immediate Addressing - generate a memory address by looking at the value of the immediate specified in the instruction
• Register Addressing - use the value stored in register as a memory address
• Base Addressing - add an immediate to a register value to create a memory address.
• PC-relative addressing - add an immediate to the program counter.

Instructions §

• A complete instruction set is found here. Pseudo-instructions here.
• Some additional standards to look out for: Multiplication, Division, and Floating Point.

Instruction Types §

• R-type

funct7 | rs2 | rs1 | funct3 | rd | opcode
  7    |  5  |  5  |   3    |  5 |   7     

• I-type

immediate | rs1 | funct3 | rd | opcode | 
   12     |  5  |    3   | 5  |   7    |

• S-type*

imm(11:5) | rs2 | rs1 | funct3 | imm(4:0) | opcode
    7     |  5  |  5  |   3   |     5     |    7  

• Shift Type*

funct6   |   immed   | rs1  | funct3 | rd | opcode 
   6     | 6 sign ext. | 5  |   3    |  5 |    7

Procedure Calling §

In order to call a procedure using RISC-V we need to do the following

1. Place the parameters in the a0 to a7 registers. This is to ensure that we do not lose any parameters.
2. Transfer control to the procedure. We do this using the JAL and the JALR instructions.
3. Acquire storage for procedure
4. Perform the procedure
5. Place the result in register for caller
6. Return to the address of the caller (i.e., the address in ra)

Links §

• Computer Organization and Architecture (RISC-V) EECS2021E by Amir Ashouri - lecture series on computer organization and architecture