RISC-V

Instructions in binary

Minimum Sized ISA

Registers

• R32, 5-Bits specify register. 32 registers
• 4-bits will get you 16 (14)
• 3-Bits will get you 8 (6)
• 2 registers are fixed. R0 is always zero, XLEN-1 (last reg) is PC.

"For resource-constrained embedded applications, we have defined the RV32E subset, which only has 16 registers (Chapter 3)."

Op Codes

• All the 32-bit instructions in the base ISA have their lowest two bits set to 11

xxxxxxxxxxxxxxaa 16-bit (aa != 11)
xxxxxxxxxxxbbb11 32-bit (bbb != 111)
xxxxxxxxxx011111 48-bit
xxxxxxxxx0111111 64-bit
xnnnxxxxx1111111 (80+16*nnn)-bit, nnn != 111
x111xxxxx1111111 Reserved for ≥192-bits

Word Size

• an 8-bit Atmel AVR Arduino, sizeof(int) returns 2 (e.g. 16-bits) when compiled with GCC 4.3.2 (WinAVR 20081205)
• int is a C type. The ATMega's have no concept of it, they work only with 8bit bytes (because they have 8bit AVR CPUs).