rv32.lua

local function implodebits(bits,count,signed)
	local negative = false
	if signed then
		negative = bits[count-1]
	end
	local out = 0
	for i=0,count-1 do
		if bits[i] then out = out + (2^i) end
	end
	if negative then out = out - (2^count) end
	return out
end

local function explodebits(num,count)
	num = num%(2^count)
	if num < 0 then
		num = num + (2^count)
	end
	local out = {}
	for i=0,count-1 do
		out[i] = num%(2^(i+1)) >= 2^i
	end
	return out
end

local function signextend(bits,fromcount,tocount)
	for i=fromcount,tocount-1 do
		bits[i] = bits[fromcount-1]
	end
end

local registers = {}

for i=0,31 do registers[i] = 0 end

registers.pc = 0

local ram = {}

local running = false

local function getreg(reg)
	return reg == 0 and 0 or registers[reg]
end

local function setreg(reg,val)
	if val < 0 then val = val + (2^32) end
	if reg ~= 0 then registers[reg] = math.floor(val%(2^32)) end
end

local function readram(address,bytes)
	local out = 0
	for i=0,bytes-1 do
		out = out + (ram[address+i] or 0) * 2^(8*i)
	end
	return out
end

local function writeram(address,data,bytes)
	for i=0,bytes-1 do
		local thisbyte = data % (2^((i+1)*8)) / 2^(i*8)
		thisbyte = math.floor(thisbyte) % 2^32
		if thisbyte ~= 0 then
			ram[address+i] = thisbyte
		else
			--nil values are turned back into 0 on read
			ram[address+i] = nil
		end
	end
end

local function printstate()
	local buf = ""
	for i=0,31 do
		buf = buf..string.format("x%02d: %08X    ",i,getreg(i))
		if i%4 == 3 then
			buf = buf.."\n"
		end
	end
	buf = buf..string.format(" PC: %08X\n",registers.pc)
	print(buf)
end

local operations = {
	add = function(rd,rs1,rs2)
		setreg(rd,getreg(rs1)+getreg(rs2))
	end,
	sub = function(rd,rs1,rs2)
		setreg(rd,getreg(rs1)-getreg(rs2))
	end,
	xor = function(rd,rs1,rs2)
		local rs1bits = explodebits(getreg(rs1),32)
		local rs2bits = explodebits(getreg(rs2),32)
		local rdbits = {}
		for i=0,31 do rdbits[i] = rs1bits[i] ~= rs2bits[i] end
		setreg(rd,implodebits(rdbits,32,false))
	end,
	orr = function(rd,rs1,rs2) --instruction is "or" but that's a reserved word
		local rs1bits = explodebits(getreg(rs1),32)
		local rs2bits = explodebits(getreg(rs2),32)
		local rdbits = {}
		for i=0,31 do rdbits[i] = rs1bits[i] or rs2bits[i] end
		setreg(rd,implodebits(rdbits,32,false))
	end,
	andr = function(rd,rs1,rs2) --instruction is "and" but that's a reserved word
		local rs1bits = explodebits(getreg(rs1),32)
		local rs2bits = explodebits(getreg(rs2),32)
		local rdbits = {}
		for i=0,31 do rdbits[i] = rs1bits[i] and rs2bits[i] end
		setreg(rd,implodebits(rdbits,32,false))
	end,
	sll = function(rd,rs1,rs2)
		setreg(rd,getreg(rs1)*(2^getreg(rs2)))
	end,
	srl = function(rd,rs1,rs2)
		setreg(rd,getreg(rs1)/(2^getreg(rs2)))
	end,
	sra = function(rd,rs1,rs2)
		local num = getreg(rs1)
		local amount = getreg(rs2)
		local sign = num>=(2^31)
		if amount == 0 then return end
		for _=1,amount do
			num = num/2
			if sign then num = num+(2^31) end
		end
		setreg(rd,num)
	end,
	slt = function(rd,rs1,rs2)
		local num1 = getreg(rs1)
		local num2 = getreg(rs2)
		if num1 >= 2^31 then num1 = num1-(2^32) end
		if num2 >= 2^31 then num2 = num2-(2^32) end
		setreg(rd,num1 < num2 and 1 or 0)
	end,
	sltu = function(rd,rs1,rs2)
		local num1 = getreg(rs1)
		local num2 = getreg(rs2)
		setreg(rd,num1 < num2 and 1 or 0)
	end,
	addi = function(rd,rs1,imm)
		local immbits = explodebits(imm,12)
		signextend(immbits,12,32)
		imm = implodebits(immbits,32,false)
		setreg(rd,getreg(rs1)+imm)
	end,
	xori = function(rd,rs1,imm)
		local rs1bits = explodebits(getreg(rs1),32)
		local immbits = explodebits(imm,12)
		signextend(immbits,12,32)
		local rdbits = {}
		for i=0,31 do
			rdbits[i] = rs1bits[i] ~= immbits[i]
		end
		setreg(rd,implodebits(rdbits,32,false))
	end,
	ori = function(rd,rs1,imm)
		local rs1bits = explodebits(getreg(rs1),32)
		local immbits = explodebits(imm,12)
		signextend(immbits,12,32)
		local rdbits = {}
		for i=0,31 do
			rdbits[i] = rs1bits[i] or immbits[i]
		end
		setreg(rd,implodebits(rdbits,32,false))
	end,
	andi = function(rd,rs1,imm)
		local rs1bits = explodebits(getreg(rs1),32)
		local immbits = explodebits(imm,12)
		signextend(immbits,12,32)
		local rdbits = {}
		for i=0,31 do
			rdbits[i] = rs1bits[i] and immbits[i]
		end
		setreg(rd,implodebits(rdbits,32,false))
	end,
	slli = function(rd,rs1,imm)
		setreg(rd,getreg(rs1)*(2^imm))
	end,
	srli = function(rd,rs1,imm)
		setreg(rd,getreg(rs1)/(2^imm))
	end,
	srai = function(rd,rs1,imm)
		local num = getreg(rs1)
		local sign = num>=(2^31)
		if imm == 0 then return end
		for _=1,imm do
			num = num/2
			if sign then num = num+(2^31) end
		end
		setreg(rd,num)
	end,
	slti = function(rd,rs1,imm)
		local num1 = getreg(rs1)
		local immbits = explodebits(imm,12)
		signextend(immbits,12,32)
		local num2 = implodebits(immbits,32,false)
		if num1 >= 2^31 then num1 = num1-(2^32) end
		if num2 >= 2^31 then num2 = num2-(2^32) end
		setreg(rd,num1 < num2 and 1 or 0)
	end,
	sltiu = function(rd,rs1,imm)
		local num1 = getreg(rs1)
		local immbits = explodebits(imm,12)
		signextend(immbits,12,32)
		local num2 = implodebits(immbits,32,false)
		setreg(rd,num1 < num2 and 1 or 0)
	end,
	beq = function(rs1,rs2,imm)
		local num1 = getreg(rs1)
		local num2 = getreg(rs2)
		if num1 == num2 then
			local immbits = explodebits(imm,13)
			signextend(immbits,13,32)
			imm = implodebits(immbits,13,true)
			registers.pc = (registers.pc + imm) % 2^32
			return true
		end
	end,
	bne = function(rs1,rs2,imm)
		local num1 = getreg(rs1)
		local num2 = getreg(rs2)
		if num1 ~= num2 then
			local immbits = explodebits(imm,13)
			signextend(immbits,13,32)
			imm = implodebits(immbits,13,true)
			registers.pc = (registers.pc + imm) % 2^32
			return true
		end
	end,
	blt = function(rs1,rs2,imm)
		local num1 = getreg(rs1)
		local num2 = getreg(rs2)
		if num1 >= 2^31 then num1 = num1-(2^32) end
		if num2 >= 2^31 then num2 = num2-(2^32) end
		if num1 < num2 then
			local immbits = explodebits(imm,13)
			signextend(immbits,13,32)
			imm = implodebits(immbits,13,true)
			registers.pc = (registers.pc + imm) % 2^32
			return true
		end
	end,
	bge = function(rs1,rs2,imm)
		local num1 = getreg(rs1)
		local num2 = getreg(rs2)
		if num1 >= 2^31 then num1 = num1-(2^32) end
		if num2 >= 2^31 then num2 = num2-(2^32) end
		if num1 >= num2 then
			local immbits = explodebits(imm,13)
			signextend(immbits,13,32)
			imm = implodebits(immbits,13,true)
			registers.pc = (registers.pc + imm) % 2^32
			return true
		end
	end,
	bltu = function(rs1,rs2,imm)
		local num1 = getreg(rs1)
		local num2 = getreg(rs2)
		if num1 < num2 then
			local immbits = explodebits(imm,13)
			signextend(immbits,13,32)
			imm = implodebits(immbits,13,true)
			registers.pc = (registers.pc + imm) % 2^32
			return true
		end
	end,
	bgeu = function(rs1,rs2,imm)
		local num1 = getreg(rs1)
		local num2 = getreg(rs2)
		if num1 >= num2 then
			local immbits = explodebits(imm,13)
			signextend(immbits,13,32)
			imm = implodebits(immbits,13,true)
			registers.pc = (registers.pc + imm) % 2^32
			return true
		end
	end,
	jal = function(rd,imm)
		setreg(rd,registers.pc+4)
		local immbits = explodebits(imm,21)
		signextend(immbits,21,32)
		imm = implodebits(immbits,32,true)
		registers.pc = (registers.pc + imm) % 2^32
		return true
	end,
	jalr = function(rd,rs1,imm)
		setreg(rd,registers.pc+4)
		local immbits = explodebits(imm,12)
		signextend(immbits,12,32)
		imm = implodebits(immbits,32,true)
		registers.pc = (getreg(rs1) + imm) % 2^32
		return true
	end,
	lui = function(rd,imm)
		setreg(rd,imm)
	end,
	auipc = function(rd,imm)
		setreg(rd,registers.pc+imm)
	end,
	--TODO: lb/lh/lw/lbu/lhu/sb/sh/sw (need memory first)
	ecall = function()
		--right now the only call is to print the machine state
		print("Software requested register dump")
		printstate()
	end,
	ebreak = function()
		--for now this just stops the processor and prints state
		running = false
		print("Hit ebreak, system halted")
		printstate()
	end,
}

local function runinst(instruction)
	local bits = explodebits(instruction,32)
	local opcode = implodebits(bits,7)
	if opcode == 0x33 then
		--R-type
		--This is spectaularly inefficient and will probably need to be optimized later.
		--Really, the whole program is.
		--Focus is on making it work first...
		local f3bits = {[0] = bits[12],bits[13],bits[14]}
		local f3 = implodebits(f3bits,3)
		local f7bits = {[0] = bits[25],bits[26],bits[27],bits[28],bits[29],bits[30],bits[31]}
		local f7 = implodebits(f7bits,7)
		local rdbits = {[0] = bits[7],bits[8],bits[9],bits[10],bits[11]}
		local rd = implodebits(rdbits,5)
		local rs1bits = {[0] = bits[15],bits[16],bits[17],bits[18],bits[19]}
		local rs1 = implodebits(rs1bits,5)
		local rs2bits = {[0] = bits[20],bits[21],bits[22],bits[23],bits[24]}
		local rs2 = implodebits(rs2bits,5)
		if f3 == 0x0 and f7 == 0x0 then
			operations.add(rd,rs1,rs2)
		elseif f3 == 0x0 and f7 == 0x20 then
			operations.sub(rd,rs1,rs2)
		elseif f3 == 0x4 and f7 == 0x0 then
			operations.xor(rd,rs1,rs2)
		elseif f3 == 0x6 and f7 == 0x0 then
			operations.orr(rd,rs1,rs2)
		elseif f3 == 0x7 and f7 == 0x0 then
			operations.andr(rd,rs1,rs2)
		elseif f3 == 0x1 and f7 == 0x0 then
			operations.sll(rd,rs1,rs2)
		elseif f3 == 0x5 and f7 == 0x0 then
			operations.srl(rd,rs1,rs2)
		elseif f3 == 0x5 and f7 == 0x20 then
			operations.sra(rd,rs1,rs2)
		elseif f3 == 0x2 and f7 == 0x0 then
			operations.slt(rd,rs1,rs2)
		elseif f3 == 0x3 and f7 == 0x0 then
			operations.sltu(rd,rs1,rs2)
		end
	elseif opcode == 0x13 or opcode == 0x3 or opcode == 0x67 or opcode == 0x73 then
		--I-type
		local f3bits = {[0] = bits[12],bits[13],bits[14]}
		local f3 = implodebits(f3bits,3)
		local rdbits = {[0] = bits[7],bits[8],bits[9],bits[10],bits[11]}
		local rd = implodebits(rdbits,5)
		local rs1bits = {[0] = bits[15],bits[16],bits[17],bits[18],bits[19]}
		local rs1 = implodebits(rs1bits,5)
		local immbits = {[0] = bits[20],bits[21],bits[22],bits[23],bits[24],bits[25],bits[26],bits[27],bits[28],bits[29],bits[30],bits[31]}
		local imm = implodebits(immbits,12)
		if opcode == 0x13 then
			if f3 == 0x0 then
				operations.addi(rd,rs1,imm)
			elseif f3 == 0x4 then
				operations.xori(rd,rs1,imm)
			elseif f3 == 0x6 then
				operations.ori(rd,rs1,imm)
			elseif f3 == 0x7 then
				operations.andi(rd,rs1,imm)
			elseif f3 == 0x1 and imm/0x20 == 0x0 then
				operations.slli(rd,rs1,imm%0x20)
			elseif f3 == 0x5 and imm/0x20 == 0x0 then
				operations.srli(rd,rs1,imm%0x20)
			elseif f3 == 0x5 and imm/0x20 == 0x20 then
				operations.srai(rd,rs1,imm%0x20)
			elseif f3 == 0x2 then
				operations.slti(rd,rs1,imm)
			elseif f3 == 0x3 then
				operations.sltiu(rd,rs1,imm)
			end
		elseif opcode == 0x3 then
			--TODO: implement lb/lh/lw/lbu/lhu
		elseif opcode == 0x67 then
			if f3 == 0x0 then
				return operations.jalr(rd,rs1,imm)
			end
		elseif opcode == 0x73 then
			if imm == 0x0 then
				operations.ecall()
			elseif imm == 0x1 then
				operations.ebreak()
			end
		end
	elseif opcode == 0x23 then
		--S-type
		local f3bits = {[0] = bits[12],bits[13],bits[14]}
		local f3 = implodebits(f3bits,3)
		local rs1bits = {[0] = bits[15],bits[16],bits[17],bits[18],bits[19]}
		local rs1 = implodebits(rs1bits,5)
		local rs2bits = {[0] = bits[20],bits[21],bits[22],bits[23],bits[24]}
		local rs2 = implodebits(rs2bits,5)
		local immbits = {[0] = bits[7],bits[8],bits[9],bits[10],bits[11],bits[25],bits[26],bits[27],bits[28],bits[29],bits[30],bits[31]}
		local imm = implodebits(immbits,12)
		--TODO: implement sb/sh/sw
	elseif opcode == 0x63 then
		--B-type
		local f3bits = {[0] = bits[12],bits[13],bits[14]}
		local f3 = implodebits(f3bits,3)
		local rs1bits = {[0] = bits[15],bits[16],bits[17],bits[18],bits[19]}
		local rs1 = implodebits(rs1bits,5)
		local rs2bits = {[0] = bits[20],bits[21],bits[22],bits[23],bits[24]}
		local rs2 = implodebits(rs2bits,5)
		local immbits = {[0] = false,bits[8],bits[9],bits[10],bits[11],bits[25],bits[26],bits[27],bits[28],bits[29],bits[30],bits[7],bits[31]}
		local imm = implodebits(immbits,13)
		if f3 == 0x0 then
			return operations.beq(rs1,rs2,imm)
		elseif f3 == 0x1 then
			return operations.bne(rs1,rs2,imm)
		elseif f3 == 0x4 then
			return operations.blt(rs1,rs2,imm)
		elseif f3 == 0x5 then
			return operations.bge(rs1,rs2,imm)
		elseif f3 == 0x6 then
			return operations.bltu(rs1,rs2,imm)
		elseif f3 == 0x7 then
			return operations.bgeu(rs1,rs2,imm)
		end
	elseif opcode == 0x37 or opcode == 0x17 then
		--U-type
		local rdbits = {[0] = bits[7],bits[8],bits[9],bits[10],bits[11]}
		local rd = implodebits(rdbits,5)
		--Immediate bits in this type actually all line up between the instruction and their actual value(!)
		--This means it's way easier to just mask out rd/opcode
		local imm = instruction - (instruction % 0x1000)
		if opcode == 0x37 then
			operations.lui(rd,imm)
		elseif opcode == 0x17 then
			operations.auipc(rd,imm)
		end
	elseif opcode == 0x6f then
		--J-type
		local rdbits = {[0] = bits[7],bits[8],bits[9],bits[10],bits[11]}
		local rd = implodebits(rdbits,5)
		--Something had to give after how easy the U-type immediates were
		local immbits = {[0] = false,bits[21],bits[22],bits[23],bits[24],bits[25],bits[26],bits[27],bits[28],bits[29],bits[30],bits[20],bits[12],bits[13],bits[14],bits[15],bits[16],bits[17],bits[18],bits[19],bits[31]}
		local imm = implodebits(immbits,21)
		return operations.jal(rd,imm)
	else
		running = false
		print(string.format("Invalid opcode %02X, system halted",opcode))
		printstate()
	end
end

local function run()
	running = true
	repeat
		local instruction = readram(registers.pc,4)
		local jumped = runinst(instruction)
		if not jumped then registers.pc = (registers.pc + 4) % 2^32 end
	until not running
end

writeram(0x00,0x00100293,4) --addi x5,x0,1
writeram(0x04,0x3e800e13,4) --addi x28,x0,1000
writeram(0x08,0x006283b3,4) --loop: add x7,x5,x6
writeram(0x0c,0x00030293,4) --addi x5,x6,0
writeram(0x10,0x00038313,4) --addi x6,x7,0
writeram(0x14,0x00000073,4) --ecall
writeram(0x18,0xffc3e8e3,4) --bltu x7,x28,loop
writeram(0x1c,0x00100073,4) --ebreak
run()