Does 64-bit RISC-V RV64 allow access to the low 32 bits of registers?

Question

I am working with a 64 bit RISC processor, and I have question can we access the lower 32 part of the 64 bit register separately? As in case of AArch64 where x0 is for entire 64-bit register and w0 corresponds to the 32 bit part of the same register.

Do we have this kind of feature in RISC-V?

Answer 1

64-bit RISC-V has 64-bit registers. Instead of using a different register name to indicate 32-bit operations, they use a different mnemonic (eg addw for a single 32-bit word width vs. add for the full register). Most(?) instructions are available in a 32-bit form as well as the full-width form.

int add(int a,int b){
    return a+b;
}

long long add64(long long a,long long b){
    return a+b;
}

Compiled by GCC10.2 for rv64gc on Godbolt

add:
        addw    a0,a0,a1
        ret
add64:
        add     a0,a0,a1
        ret

See also Does it makes sense for a clean-slate 64-bit instruction set only have 64-bit registers, if we don't care about backwards compat? re: how it's just a matter of asm syntax for MIPS/RISC-V with different mnemonics vs. AArch64 with reg names.

(Although having special mnemonics makes it clearer that not every 64-bit instruction necessarily has a 32-bit operand-size version. eg auipc doesn't need a form that truncates an address to 32-bit.)

See also Why did RV64 introduce new opcodes for 32-bit operations instead of the 64-bit ones re: the design decision. Further related:

• RV64 with RV32 instructions support
• RISC-V 32/64-bit compatibility issues