RandomX

RandomX ("random ex") is an experimental proof of work (PoW) algorithm that uses random code execution to achieve ASIC resistance.

RandomX uses a simple low-level language (instruction set), which was designed so that any random bitstring forms a valid program.

Software implementation details and design notes are written in italics.

Virtual machine

RandomX is intended to be run efficiently and easily on a general-purpose CPU. The virtual machine (VM) which runs RandomX code attempts to simulate a generic CPU using the following set of components:

DRAM

The VM has access to 4 GiB of external memory in read-only mode. The DRAM memory blob is generated from the hash of the previous block using AES encryption (TBD). The contents of the DRAM blob change on average every 2 minutes. The DRAM blob is read with a maximum rate of 2.5 GiB/s per thread.

CPUs without hardware AES support can use a GPU to generate the DRAM blob quickly. Dual channel DDR4 memory has enough bandwidth to support up to 16 mining threads.

MMU

The memory management unit (MMU) interfaces the CPU with the DRAM blob. The purpose of the MMU is to translate the random memory accesses generated by the random program into a DRAM-friendly access pattern, where memory reads are not bound by access latency. The MMU accepts a 32-bit address addr and outputs a 64-bit value from DRAM. The DRAM blob is read mostly sequentially. After an average of 8192 sequential reads, a random read is performed. An average program reads a total of 4 MiB of DRAM and has 64 random reads.

The MMU uses two internal registers:

• ma - Address of the next quadword to be read from memory (32-bit, 8-byte aligned).
• mx - A 32-bit counter that determines if the next read is sequential or random. After each read, the read address is mixed with the counter: mx ^= addr. If the right-most 13 bits of the register are zero: (mx & 0x1FFF) == 0, the value of the mx register is copied into register ma.

When the value of the ma register is changed to a random address, the memory location can be preloaded into CPU cache using the x86 PREFETCH instruction or ARM PRFM instruction. Implicit prefetch should ensure that sequentially accessed memory is already in the cache.

Scratchpad

The VM contains a 256 KiB scratchpad, which is accessed randomly both for reading and writing. The scratchpad is split into two segments (16 KiB and 240 KiB). 75% of accesses are into the first 16 KiB.

The scratchpad access pattern mimics the usual CPU cache structure. The first 16 KiB should be covered by the L1 cache, while the remaining accesses should hit the L2 cache. In some cases, the read address can be calculated in advance, which should limit the impact of L1 cache misses.

Program

The actual program is stored in a 8 KiB ring buffer structure. Each program consists of 512 random 128-bit instructions. The ring buffer structure makes sure that the program forms a closed infinite loop.

For high-performance mining, the program should be translated directly into machine code. The whole program should fit into the L1 instruction cache and hot execution paths should stay in the µOP cache that is used by newer x86 CPUs. This should limit the number of front-end stalls and keep the CPU busy most of the time.

Control unit

The control unit (CU) controls the execution of the program. It reads instructions from the program buffer and sends commands to the other units. The CU contains 3 internal registers:

• pc - Address of the next instruction in the program buffer to be executed (64-bit, 8 byte aligned).
• sp - Address of the top of the stack (64-bit, 8 byte aligned).
• ic - Instruction counter contains the number of instructions to execute before terminating. The register is decremented after each instruction and the program execution stops when ic reaches 0.

Fixed number of executed instructions should ensure roughly equal runtime of each random program.

Stack

To simulate function calls, the VM uses a stack structure. The program interacts with the stack using the CALL and RET instructions. The stack has unlimited size and each stack element is 64 bits wide.

Although there is no explicit limit of the stack size, the maximum theoretical size of the stack is 16 MiB for a program that contains only unconditional CALL instructions (the probability of randomly generating such program is about 5×10^-912). In reality, the stack size will rarely exceed 1 MiB.

Register file

The VM has 8 integer registers r0-r7 and 8 floating point registers f0-f7. All registers are 64 bits wide.

The number of registers is low enough so that they can be stored in actual hardware registers on most CPUs.

ALU

The arithmetic logic unit (ALU) performs integer operations. The ALU can perform binary integer operations from 7 groups (addition, subtraction, multiplication, division, bitwise operations, shift, rotation) with operand sizes of 64 or 32 bits.

FPU

The floating-point unit performs IEEE-754 compliant math using 64-bit double precision floating point numbers. Five basic operations are available: addition, subtraction, multiplication, division and square root.

Endianness

The VM stores and loads all data in little-endian byte order.

Instruction set

The 128-bit instruction is encoded as follows:

All flags are aligned to an 8-bit boundary for easier decoding.

Opcode

There are 256 opcodes, which are distributed between various operations depending on their weight (how often they will occur in the program on average). The distribution of opcodes is following:

operation	number of opcodes
ALU operations	142	55.5%
FPU operations	82	32.0%
Control flow	32	12.5%

Operand A

The first operand is read from memory. The location is determined by the loc(a) flag:

loc(a)[2:0]	read A from	address size (W)
000	DRAM	32 bits
001	DRAM	32 bits
010	DRAM	32 bits
011	DRAM	32 bits
100	scratchpad	15 bits
101	scratchpad	11 bits
110	scratchpad	11 bits
111	scratchpad	11 bits

Flag reg(a) encodes an integer register r0-r7. The read address is calculated as:

reg(a) ^= addr0
addr(a) = reg(a)[W-1:0]

For reading from the scratchpad, addr(a) is multiplied by 8 for 8-byte aligned access.

Operand B

The second operand is loaded either from a register or from an immediate value encoded within the instruction. The reg(b) flag encodes an integer register (ALU operations) or a floating point register (FPU operations).

loc(b)[2:0]	read B from
000	register reg(b)
001	register reg(b)
010	register reg(b)
011	register reg(b)
100	register reg(b)
101	register reg(b)
110	imm0 or imm1
111	imm0 or imm1

imm0 is an 8-bit immediate value, which is used for shift and rotate ALU operations.

imm1 is a 32-bit immediate value which is used for most operations. For operands larger than 32 bits, the value is zero-extended for unsigned instructions and sign-extended for signed instructions. For FPU instructions, the value is first left-shifted by 32 bits, treated as a signed 64-bit integer and converted to a double precision floating point format.

Operand C

The third operand is the location where the result is stored.

loc(c)[2:0]	write C to	address size (W)
000	scratchpad	15 bits
001	scratchpad	11 bits
010	scratchpad	11 bits
011	scratchpad	11 bits
100	register reg(c)	-
101	register reg(c)	-
110	register reg(c)	-
111	register reg(c)	-

The reg(c) flag encodes an integer register (ALU operations) or a floating point register (FPU operations). For writing to the scratchpad, an integer register is always used and the write address is calculated as:

addr(c) = (addr1 ^ reg(c))[W-1:0] * 8

CPUs are typically designed for a 2:1 load:store ratio, so each VM instruction performs on average 1 memory read and 0.5 write to memory.

imm0

An 8-bit immediate value that is used as the shift/rotate count by some ALU instructions and as the jump offset of the CALL instruction.

addr0

A 32-bit address mask that is used to calculate the read address for the A operand.

addr1

A 32-bit address mask that is used to calculate the write address for the C operand. addr1 is equal to imm1.

ALU instructions

weight	instruction	signed	A width	B width	C	C width
16	ADD_64	no	64	64	A + B	64
8	ADD_32	no	32	32	A + B	32
16	SUB_64	no	64	64	A - B	64
8	SUB_32	no	32	32	A - B	32
7	MUL_64	no	64	64	A * B	64
7	MULH_64	no	64	64	A * B	64
7	MUL_32	no	32	32	A * B	64
7	IMUL_32	yes	32	32	A * B	64
7	IMULH_64	yes	64	64	A * B	64
1	DIV_64	no	64	32	A / B	32
1	IDIV_64	yes	64	32	A / B	32
4	AND_64	no	64	64	A & B	64
3	AND_32	no	32	32	A & B	32
4	OR_64	no	64	64	A | B	64
3	OR_32	no	32	32	A | B	32
4	XOR_64	no	64	64	A ^ B	64
3	XOR_32	no	32	32	A ^ B	32
6	SHL_64	no	64	6	A << B	64
6	SHR_64	no	64	6	A >> B	64
6	SAR_64	yes	64	6	A >> B	64
9	ROL_64	no	64	6	A <<< B	64
9	ROR_64	no	64	6	A >>> B	64

32-bit operations

Instructions ADD_32, SUB_32, AND_32, OR_32, XOR_32 only use the low-order 32 bits of the input operands. The result of these operations is 32 bits long and bits 32-63 of C are zero.

Multiplication

There are 5 different multiplication operations. MUL_64 and MULH_64 both take 64-bit unsigned operands, but MUL_64 produces the low 64 bits of the result and MULH_64 produces the high 64 bits. MUL_32 and IMUL_32 use only the low-order 32 bits of the operands and produce a 64-bit result. The signed variant interprets the arguments as signed integers. IMULH_64 takes two 64-bit signed operands and produces the high-order 64 bits of the result.

The following table shows an example of the output of the 5 multiplication instructions for inputs A = 0xBC550E96BA88A72B and B = 0xF5391FA9F18D6273:

instruction	A interpreted as	B interpreted as	result C
MUL_64	13570769092688258859	17670189427727360627	0x28723424A9108E51
MULH_64	13570769092688258859	17670189427727360627	0xB4676D31D2B34883
MUL_32	3129517867	4052574835	0xB001AA5FA9108E51
IMUL_32	-1165449429	-242392461	0x03EBA0C1A9108E51
IMULH_64	-4875974981021292757	-776554645982190989	0x02D93EF1269D3EE5

Division

For the division instructions, the dividend is 64 bits long and the divisor 32 bits long. The IDIV_64 instruction interprets both operands as signed integers. In case of division by zero or signed overflow, the result is equal to the dividend A.

Division by zero can be handled without branching by conditional move (IF B == 0 THEN B = 1). Signed overflow happens only for the signed variant when the minimum negative value is divided by -1. In this extremely rare case, ARM produces the "correct" result, but x86 throws a hardware exception, which must be handled.

Shift and rotate

The shift/rotate instructions use just the bottom 6 bits of the B operand (imm0 is used as the immediate value). All treat A as unsigned except SAR_64, which performs an arithmetic right shift by copying the sign bit.

FPU instructions

weight	instruction	C
22	FADD	A + B
22	FSUB	A - B
22	FMUL	A * B
8	FDIV	A / B
6	FABSQRT	sqrt(A)
2	FROUND	A

FPU instructions conform to the IEEE-754 specification, so they must give correctly rounded results. Initial rounding mode is RN (Round to Nearest). Denormal values may not be produced by any operation.

Denormals can be disabled by setting the FTZ flag in x86 SSE and ARM Neon engines. This is done for performance reasons.

Operands loaded from memory are treated as signed 64-bit integers and converted to double precision floating point format. Operands loaded from floating point registers are used directly.

FABSQRT

The sign bit of the FABSQRT operand is always cleared first, so only non-negative values are used.

In x86, the SQRTSD instruction must be used. The legacy FSQRT instruction doesn't produce correctly rounded results in all cases.

FROUND

The FROUND instruction changes the rounding mode for all subsequent FPU operations depending on the two right-most bits of A:

A[1:0]	rounding mode
00	Round to Nearest (RN) mode
01	Round towards Minus Infinity (RM) mode
10	Round towards Plus Infinity (RP) mode
11	Round towards Zero (RZ) mode

The two-bit flag value exactly corresponds to bits 13-14 of the x86 MXCSR register and bits 23 and 22 (reversed) of the ARM FPSCR register.

Control flow instructions

The following 2 control flow instructions are supported:

weight	instruction	function
17	CALL	near procedure call
15	RET	return from procedure

Both instructions are conditional in 75% of cases. The jump is taken only if B <= imm1. For the 25% of cases when B is equal to imm1, the jump is unconditional. In case the branch is not taken, both instructions become "arithmetic no-op" C = A.

CALL

Taken CALL instruction pushes the values A and pc (program counter) onto the stack and then performs a forward jump relative to the value of pc. The forward offset is equal to 16 * (imm0[6:0] + 1). Maximum jump distance is therefore 128 instructions forward (this means that at least 4 correctly spaced CALL instructions are needed to form a loop in the program).

RET

The RET instruction behaves like "not taken" when the stack is empty. Taken RET instruction pops the return address raddr from the stack (it's the instruction following the previous CALL), then pops a return value retval from the stack and sets C = A ^ retval. Finally, the instruction jumps back to raddr.

Proof of work

Hash functions

Blake2b

The primary cryptographically secure hash function used by RandomX is Blake2b with an output size of 256 bits. Blake2b was specifically designed to be fast in software, especially on modern 64-bit processors, where it's around three times faster than SHA-3 and can run at a speed of around 3 clock cycles per byte of input.

Blake2b(X) refers to the 256-bit plain hash and Blake2b(K, X) refers to the 256-bit keyed hash.

HighwayHash

HighwayHash is a fast keyed pseudorandom function, which can take advantage of SIMD instructions available in modern CPUs. It's used to calculate the scratchpad digest. HighwayHash can run at a speed of about 0.3 clocks per byte using SSE 4.1.

The function is called as HighwayHash(K, X), where K is a 256-bit key.

Pseudo-random number generator

RandomX uses a permuted congruential generator (PCG) for VM initialization. A minimal C implementation is available here. The generator has an internal state of 64 bits and additional 63 bits are used to select the output stream. The generator produces 32 random bits per call.

DRAM blob initialization

TBD

VM initialization

Scratchpad initialization

The scratchpad is initialized by copying a 256 KiB block from the DRAM blob. The starting offset of the block is 262144 * i, where i is a 14-bit input parameter.

Pseudocode:

# initializes the scratchpad
def InitializeScratchpad(i):
    memcpy(Scratchpad, DRAM + 262144 * i, 262144)

Program initialization

The program is initialized from a 256-bit seed value S.

1. The PCG random number generator is initialized with state S[63:0] and increment S[127:64] | 1 (odd number).
2. The generator is used to generate 8324 random bytes.
3. The integer registers r0-r7 are initialized with bytes 0-63.
4. Floating point registers f0-f7 are initialized with bytes 64-127 interpreted as 8 64-bit signed integers converted to a double precision floating point format.
5. The program buffer is initialized with bytes 128-8319.
6. The initial value of the ma register is set to bytes 8320-8323, XORed with S[159:128] and the last 3 bits are cleared (8-byte aligned).
7. The value of the mx register is initialized as S[191:160].
8. The remaining registers are initialized with constant values: pc = 0, sp = 0, ic = 1048576.

Pseudocode:

# S is a 256-bit seed value
# initializes the program buffer and registers
def InitializeProgram(S):
    rng = Pcg32(S[63:0], S[127:64] | 1)
    a = []
    loop 2081 times:
        a.append(rng.next())
    r0 = a[0..1]
    r1 = a[2..3]
    r2 = a[4..5]
    r3 = a[6..7]
    r4 = a[8..9]
    r5 = a[10..11]
    r6 = a[12..13]
    r7 = a[14..15]
    f0 = double(a[16..17])
    f1 = double(a[18..19])
    f2 = double(a[20..21])
    f3 = double(a[22..23])
    f4 = double(a[24..25])
    f5 = double(a[26..27])
    f6 = double(a[28..29])
    f7 = double(a[30..31])
    ProgramBuffer = a[32..2079]
    ma = (a[2080] ^ S[159:128]) & 0xFFFFFFF8
    mx = S[191:160]
    pc = 0
    sp = 0
    ic = 1048576

PoW hash calculation

RandomX produces a 256-bit final hash value to be used for a Hashcash-style proof evaluation. The hash of the input (block header for a cryptocurrency) is used for the first VM initiazation. The program initialization and program execution are chained three times to discourage mining strategies that search for programs with particular properties. The scratchpad is preserved between the 3 program executions.

Pseudocode:

# H is the input value
# returns a 256-bit PoW hash
def RandomXPoW(H):
    K = Blake2b(H)
    InitializeScratchpad(K[205:192])
    S = K
    loop 3 times:
        InitializeProgram(S)
        ExecuteProgram()
        S = Blake2b(K, RegisterFile)
    W = HighwayHash(K, Scratchpad)
    return Blake2b(K, RegisterFile + W)

The stack is not included in the result calculation to enable platform-specific return addresses. An average program takes roughly 2.5 ms to execute on a recent CPU (preliminary tests). VM initialization and result calculation should take less than 0.1 ms. The total time to calculate the PoW should be under 10 ms (depends on the overhead of translating RandomX code into machine code).

Test code

A python generator is available to generate a random program and output its C source code.

Generate a random program:

python rx2c.py > rx-sample.c

Compile the program:

gcc -O2 -maes -DRAM -DPREF rx-sample.c -o rx-sample

(Note that the test program can be compiled only by the GCC compiler due to the use of non-standard C features such as computed goto.)

Run the program:

./rx-sample

(Note that the test program execution requires more than 4 GiB of available virtual memory and the AES-NI instruction set support.)