Differences between 32 bit and 64 bit modes

Unfortunately, 64 bit RISC-V is not directly compatible with 32 bit RISC-V. The semantics of add, and other arithmetic instructions change to work on the whole 64 bit registers rather than just the 32 bits that the 32 bit version would operate on. The "w" suffix instructions, mimic the behavior of the 32 bit versions of instructions and sign extend the top 32 bits.

Shifts are also a little different. Shifts consider 1 more bit to determine how far to shift. This allows them to shift an extra 32 bits. Immediate shifts allow encoding an additional bit as well.

64 bit only instructions

Example Usage	Description
addiw t1,t2,-100	Addition immediate: set t1 to (t2 plus signed 12-bit immediate) using only the lower 32 bits
addw t1,t2,t3	Addition: set t1 to (t2 plus t3) using only the lower 32 bits
divuw t1,t2,t3	Division: set t1 to the result of t2/t3 using unsigned division limited to 32 bits
divw t1,t2,t3	Division: set t1 to the result of t2/t3 using only the lower 32 bits
fcvt.d.l f1, t1, dyn	Convert double from long: Assigns the value of t1 to f1
fcvt.d.lu f1, t1, dyn	Convert double from unsigned long: Assigns the value of t1 to f1
fcvt.l.d t1, f1, dyn	Convert 64 bit integer from double: Assigns the value of f1 (rounded) to t1
fcvt.l.s t1, f1, dyn	Convert 64 bit integer from float: Assigns the value of f1 (rounded) to t1
fcvt.lu.d t1, f1, dyn	Convert unsigned 64 bit integer from double: Assigns the value of f1 (rounded) to t1
fcvt.lu.s t1, f1, dyn	Convert unsigned 64 bit integer from float: Assigns the value of f1 (rounded) to t1
fcvt.s.l f1, t1, dyn	Convert float from long: Assigns the value of t1 to f1
fcvt.s.lu f1, t1, dyn	Convert float from unsigned long: Assigns the value of t1 to f1
fmv.d.x f1, t1	Move float: move bits representing a double from an 64 bit integer register
fmv.x.d t1, f1	Move double: move bits representing a double to an 64 bit integer register
ld t1, -100(t2)	Set t1 to contents of effective memory double word address
lwu t1, -100(t2)	Set t1 to contents of effective memory word address without sign-extension
mulw t1,t2,t3	Multiplication: set t1 to the lower 32 bits of t2*t3 using only the lower 32 bits of the input
remuw t1,t2,t3	Remainder: set t1 to the remainder of t2/t3 using unsigned division limited to 32 bits
remw t1,t2,t3	Remainder: set t1 to the remainder of t2/t3 using only the lower 32 bits
sd t1, -100(t2)	Store double word : Store contents of t1 into effective memory double word address
slli t1,t2,33	Shift left logical : Set t1 to result of shifting t2 left by number of bits specified by immediate
slliw t1,t2,10	Shift left logical (32 bit): Set t1 to result of shifting t2 left by number of bits specified by immediate
sllw t1,t2,t3	Shift left logical (32 bit): Set t1 to result of shifting t2 left by number of bits specified by value in low-order 5 bits of t3
srai t1,t2,33	Shift right arithmetic : Set t1 to result of sign-extended shifting t2 right by number of bits specified by immediate
sraiw t1,t2,10	Shift right arithmetic (32 bit): Set t1 to result of sign-extended shifting t2 right by number of bits specified by immediate
sraw t1,t2,t3	Shift left logical (32 bit): Set t1 to result of shifting t2 left by number of bits specified by value in low-order 5 bits of t3
srli t1,t2,33	Shift right logical : Set t1 to result of shifting t2 right by number of bits specified by immediate
srliw t1,t2,10	Shift right logical (32 bit): Set t1 to result of shifting t2 right by number of bits specified by immediate
srlw t1,t2,t3	Shift left logical (32 bit): Set t1 to result of shifting t2 left by number of bits specified by value in low-order 5 bits of t3
subw t1,t2,t3	Subtraction: set t1 to (t2 minus t3) using only the lower 32 bits

64 bit only psuedo-instructions:

Example Usage	Description
fcvt.d.l f1, t1	Convert double from signed 64 bit integer: Assigns the value of t1 to f1
fcvt.d.lu f1, t1	Convert double from unsigned 64 bit integer: Assigns the value of t1 to f1
fcvt.l.d t1, f1	Convert signed 64 bit integer from double: Assigns the value of f1 (rounded) to t1
fcvt.l.s t1, f1	Convert signed 64 bit integer from float: Assigns the value of f1 (rounded) to t1
fcvt.lu.d t1, f1	Convert unsigned 64 bit integer from double: Assigns the value of f1 (rounded) to t1
fcvt.lu.s t1, f1	Convert unsigned 64 bit integer from float: Assigns the value of f1 (rounded) to t1
fcvt.s.l f1, t1	Convert float from signed 64 bit integer: Assigns the value of t1 to f1
fcvt.s.lu f1, t1	Convert float from unsigned 64 bit integer: Assigns the value of t1 to f1
li t1,1000000000000000	Load Immediate : Set t1 to 64-bit immediate