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RISC-V vstart / vstart Register: Vector Start CSR

vstart specifies the first element executed by a vector instruction. Hardware normally writes it on a trap; vector instructions reset it to zero on completion. The psABI allows procedures to assume zero on entry and return.

ABI name: vstart; physical register: vstart.

Save rule: A procedure that writes nonzero clears it before return or call.

Read it through its psABI role first, then inspect how each instruction reads or writes it.

Physical Name vstartABI Name vstartSave Rule A procedure that writes nonzero clears it before return or call

Role

Vector Start CSR

Convention

psABI: zero on procedure entry and return

Remember This First

The calling convention listed for this entry is: psABI: zero on procedure entry and return.

RISC-V Vector ISA Reference

ABI register

Preserved across calls: A procedure that writes nonzero clears it before return or call

RISC-V Unprivileged ISA Manual, V extension

When It Fits Best

- Use it according to the documented role: Vector Start CSR.
- Consult the applicable ISA or ABI specification for concrete read, write, or execution semantics.

When Not To Use It This Way

- Do not apply the integer temporary-register t* rules to this entry.
- Do not infer behavior not defined by this page or the applicable specification from the ABI name.

What Happens Around A Call

The calling convention listed for this entry is: psABI: zero on procedure entry and return.

The save rule listed for this entry is: A procedure that writes nonzero clears it before return or call.

Read The Documented Role

Examples explain the rule, not a complete program

# vstart / vstart
# Vector Start CSR
# psABI: zero on procedure entry and return