RISC-V-Vector-Processor

256-bit vector processor based on the RISC-V vector (V) extension Currently, only the execution units are published, however, the registers, fetch and decode units are in development. THIS PROJECT IS IN ACTIVE DEVELOPMENT AND SHOULD NOT BE CONSIDERED BUG FREE

1. Background

2. RISC-V Vector Extension Terminology

2.1 Standard Element Width (SEW)

The SEW defines the width of each word in the vector. For example, a 256-bit vector could contain 8 32-bit words, in this case, the SEW would be 32 bits. SEW is encoded as a 3-bit value using the following method:

sew/vsew	SEW
000	8
001	16
010	32
011	64
100	128
101	256
110	512
111	1024

Although some execution units can operate on and value of SEW up to 256 (bits), most only support up to 64 (bits).

2.2 Vector Length (VLEN)

VLEN describes the total length (usually in bits) of the vectors operated upon by the processor. In this design, VLEN is fixed at 256 bits. In the future, some simple elements, such as the ALU may be extended to support arbitrary vector lengths.

3. Features

3.1 Integer Addition/Subtraction

Addition and subtraction are performed using the addsub_256bit module (addsub.sv). This can be used on its own or combined with the logic unit as the ALU module, vector_alu (vector_alu.sv).

Port	Direction	Width	Description
vaddsub_en_i	in	1	Active high enable
a_i	in	256	Vector input A
b_i	in	256	Vector input B
sew_i	in	3	Standard element width (8, 16, 32, 64, 128, 256)
carry_ext_i	in	32	External carry/borrow in
op_i	in	1	Operation: 0 = subtract, 1 = add
out_o	out	256	Vector output
cout_o	out	32	Carry/borrow out

3.2 Logic

Logic and shift operations are performed by the logic_256bit module (vector_logic.sv). This can be used on its own or combined with the addsub unit as the ALU module, vector_alu (vector_alu.sv).

Port	Direction	Width	Description
a_i	in	256	Vector input A
b_i	in	256	Vector input B
sew_i	in	3	Standard element width (8, 16, 32, 64, 128, 256)
opcode_i	in	6	Logic/ALU opcodes
carry_ext_i	in	32	External carry/borrow in

3.3 Integer Multiplication

Integer multiplication is performed by the mult_256bit module (vector_mult.sv). As the design targets Xilinx FPGAs, the (* use_dsp48 = "true" *) directive is added to force the tool to infer DSP48 blocks to perform the multiplication. This can be removed if targeting for simulation or if a non-Xilinx FPGA is used.

Port	Direction	Width	Description
a_i	in	256	Vector input A
b_i	in	256	Vector input B
sew_i	in	3	Standard element width (8, 16, 32, 64)
out_o	out	256	Vector output

3.4 Shift

3.5 Integer Compare

Currently in development

3.6 Vector Masking

Vector masking allows certain elements of an input vector to be ignored by a execution unit. Vector register v0 is used as the vector mask register, each element in a vector is allocated a single bit in the mask register. Element i is masked by bit i in the mask register. Currently, only the ALU (module vector_alu) supports vector masking.

4. Processor Opcodes

Please note, these are the opcodes used for specifying the operation of each execution unit and are not identical to the opcodes used by the RISC-V vector extension standard. The decode module (not yet published) is responsible for this conversion.

4.1 ALU

Binary Code	Opcode	Operation
000000	ALU_VAND	A & B
000001	ALU_VNAND	¬(A & B)
000010	ALU_VANDNOT	A & ¬B
000011	ALU_VOR	A + B
000100	ALU_VNOR	¬(A + B)
000101	ALU_VXOR	A ⊕ B
000110	ALU_VXNOR	¬(A ⊕ B)
000111	ALU_VNOT	¬A
001000	ALU_VSLL	A << B
001001	ALU_VSRL	A >> B
001010	ALU_VSRA	A >>> B
001011	ALU_VADD	A + B
001100	ALU_VSUB	A - B
001101	ALU_VMIN	min(A, B)
001110	ALU_VMAX	max(A, B)
001111	ALU_VADC	A + B + Cin
001110	ALU_VSBC	A - B - Cin