英文:
Why the risc-v instruction "addi sp,sp,-32" is converted to binary code "11 01"?
问题
"addi sp,sp,-40" 转换成二进制代码为 "fd810113"。
"addi sp,sp,-32" 转换成二进制代码为 "1101"。
英文:
I'm learning risc-v instruction set. I know that "addi sp,sp,-40" will be converted to "fd810113". But I don't know why "addi sp,sp,-32" is converted to binary code "11 01".
答案1
得分: 3
这是使用压缩指令格式的部分：
C.ADDI[2:0] nzimm[5] dest[4:0] nzimm[4:0] C1[1:0]
当C.ADDI为000，SP为寄存器#2，C1为01时，得到：
000 1 00010 00000 01 = 0001 0001 0000 0001 = 0x1101
请注意，由于只有6位空间，压缩指令只接受来自-32到+31的立即数，这就是为什么另一条立即数为-40的指令使用32位格式的原因。
英文:
That is using the compressed instruction format:
C.ADDI[2:0] nzimm[5] dest[4:0] nzimm[4:0] C1[1:0]
With C.ADDI being 000, SP being register #2 and C1 being 01 it gives:
000 1 00010 00000 01 = 0001 0001 0000 0001 = 0x1101
Note that the compressed instruction only takes immediate from -32 to +31 due to having 6 bits of space which is why the other instruction with -40 is using the 32 bit format.
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