1. Tech-Vector-Ext
  2. Topics

The Width of vcsr and vstart

Tianyi Xia <tianshi.xty@...>
 

Hi,all
Debuggers can use abstract commands to access CSRs. When using abstract commands, debuggers need to specify the bit width of CSRs. The bit widths of vcsr and vstart are not clearly defined in Vector Extension version1.0. In an RV64 implementation, The debugger is not clear whether the bit width of these two CSRs should be regarded as 32 or 64. May be we need specify the bit width of these CSRs in the spec, XLEN or fixed-length 32bit.

 The fcsr defined in RISC-V Unprivileged ISA is fixed-length 32bit CSR. The register structure of vcsr is similar to fcsr.So maybe vcsr should also be defined as a fixed-length 32bit register?

 

Thanks,

 

Tianyi Xia
andrew@...
 

For the current V extension, it's correct to treat both vcsr and vstart as 32-bit registers.  I agree the spec should clearly indicate whether or not these registers will always be 32 bits (like fcsr).


On Wed, Dec 15, 2021 at 6:31 PM Tianyi Xia via lists.riscv.org <tianshi.xty=alibaba-inc.com@...> wrote:

Hi,all
Debuggers can use abstract commands to access CSRs. When using abstract commands, debuggers need to specify the bit width of CSRs. The bit widths of vcsr and vstart are not clearly defined in Vector Extension version1.0. In an RV64 implementation, The debugger is not clear whether the bit width of these two CSRs should be regarded as 32 or 64. May be we need specify the bit width of these CSRs in the spec, XLEN or fixed-length 32bit.

 The fcsr defined in RISC-V Unprivileged ISA is fixed-length 32bit CSR. The register structure of vcsr is similar to fcsr.So maybe vcsr should also be defined as a fixed-length 32bit register?

 

Thanks,

 

Tianyi Xia
Krste Asanovic
 

Thanks for spotting the oversight. 
The spec was updated to indicate these should be treated as XLEN-bit wide registers.

There is no effective difference right now given that upper bits are not currently defined, but there may be some use for >32 bits in vcsr in some distant future. Vstart could also acquire extra exception state in some distant future.

Krste


On Dec 16, 2021, at 1:32 AM, Andrew Waterman <andrew@...> wrote:

For the current V extension, it's correct to treat both vcsr and vstart as 32-bit registers.  I agree the spec should clearly indicate whether or not these registers will always be 32 bits (like fcsr).


On Wed, Dec 15, 2021 at 6:31 PM Tianyi Xia via lists.riscv.org <tianshi.xty=alibaba-inc.com@...> wrote:
Hi,all
Debuggers can use abstract commands to access CSRs. When using abstract commands, debuggers need to specify the bit width of CSRs. The bit widths of vcsr and vstart are not clearly defined in Vector Extension version1.0. In an RV64 implementation, The debugger is not clear whether the bit width of these two CSRs should be regarded as 32 or 64. May be we need specify the bit width of these CSRs in the spec, XLEN or fixed-length 32bit.
 The fcsr defined in RISC-V Unprivileged ISA is fixed-length 32bit CSR. The register structure of vcsr is similar to fcsr.So maybe vcsr should also be defined as a fixed-length 32bit register?

 

Thanks,

 

Tianyi Xia

1 - 3 of 3