Fact Sheet on Registers/Instructions
Machines typically 16 or 32 registers
More registers => larger instruction format
More registers => possibly slower clock speed
Register 0 is the constant 0 in MIPS ISA
Extending the instruction set ...
sub $8, $0, $8 # negate $8
add $9, $0, $5 # move $5 to $9
add $5, $0, $0 # set $8 to zero