06.3 Integer computations

Types of operations:

• Integer register-register operations (R-type)
• Integer register-immediate operations (I-type/U-type)
• No-operation (NOP)

Int Register-Register

Executes an operation on data from two registers (source) and stores it in a new register (destination) from the register file (RF)

RF[rd] = f(RF[rs1], RF[rs2])

It is formatted over 32 bits as such:

funct7	rs2	rs1	funct3	rd	opcode
7	5	5	3	5	7
7bit function code	source index	source index	3 bit function code	destination index	7bit operation code OP

Example

For xor of t1 and t2 and stored into t0:

• 0000000 - funct7
• 00111 - rs2
• 00110 - rs1
• 100 - funct3
• 00101 - rd
• 0110011 - opcode

Which gives: 0x007342B3 as the 32bit operation

We have the operations:

• ADD/SUB — addition/subtraction
• AND/OR/XOR — bitwise logical operations
• SLL/SRL/SRA — logical left/right/arithmetic shift
• SLT/SLTU — (un)signed comparisons

Example

Let’s do $a=b+c+d-e$ in risc-v:

# add b and c into a
add t0, t1, t2
# add d to b + c into a
add t0, t0, s0
# subtract e to a into a
sub t0, t0, s1

Int Register-Immediate

Immediate ⇐> integer literal

Can be signed or unsigned

I type

I-type

Is in the format (Except for shifting):

imm	rs1	funct3	rd	opcode
12	5	3	5	7
12-bit immediate	source index	3bit function	destination index	operation code OP-IMM

For shifting we have:

funct7	imm	rs1	funct3	rd	opcode
7	5	5	3	5	7
7 bit function	5-bit immediate	source index	3 bit function	destination index	operation code OP-IMM

U type

U-type — Upper Immediate

Is in the format:

imm	rd	opcode
20	5	7
20-bit immediate	destination index	operation code

With those two types of immediate operations, we can load a full 32bit number

NOP

“No operation”

Just advances program counter

It is the equivalent of addi x0, x0, 0

06.4 Memory