update

README.txt

@@ -2,3 +2,18 @@ This is just for me to understand how all this works, and to learn something new
 
 For example, I learned that endianness is not per bit but per byte. My whole life I've been in misinformation.
 So little endian is not that first bit is the LSB, but it's MSB... and after 8 bits it becomes larger, which makes no point
+
+And I learned about sign extension, which is pretty cool
+
+And I learned that Java is bad because it doesn't have unsigned numbers
+
+To compile stuff:
+0. Get the toolchain obviously
+1. riscv32-unknown-elf-gcc -c -Oz program.c
+2. riscv32-unknown-elf-objcopy -O binary program.o program.bin
+   program.bin is the binary file with the program
+3. Encode to Base64: cat program.bin | base64
+
+rv32i, ilp32 compatible toolchain for 64bit Linux: https://lfs.m724.eu/toolchain.tar.zst
+Or just the stuff necessary to make a binary file: https://lfs.m724.eu/toolchainlite.tar.zst
+Those were compiled with `./configure --prefix=$(pwd)/../toolchain --with-arch=rv32i --with-abi=ilp32` and `make`

src/main/java/eu/m724/AddressSpace.java

@@ -1,8 +1,5 @@
 package eu.m724;
 
-import java.util.Arrays;
-import java.util.Base64;
-
 /**
  * Represents an address space
  *
@@ -31,40 +28,33 @@ public class AddressSpace {
         if (isRomInit) throw new RuntimeException("ROM already initialized");
         isRomInit = true;
 
-        for (int i=0; i<program.length; i++) {
+        if (program.length > romSize) {
-            rom[i] = program[i];
+            throw new RuntimeException("Program is larger than ROM");
         }
+
+        System.arraycopy(program, 0, rom, 0, program.length);
     }
 
     /**
-     * read {@code n} bytes from ROM<br>
+     * read {@code n} bytes<br>
      *
      * @param address address of first byte
      * @param n number of bytes, max 8
      * @return the unsigned bytes
      */
-    long readRom(int address, int n) {
+    long read(int address, int n) {
         long v = 0;
 
-        for (int i=0; i<n; i++) {
+        if (address < romSize) {
-            v |= (long) (rom[address + i] & 0xFF) << (8 * i);
+            for (int i=0; i<n; i++) {
-        }
+                v |= (long) (rom[address + i] & 0xFF) << (8 * i);
-
+            }
-        return v;
+        } else if (address < romSize + ramSize) {
-    }
+            for (int i=0; i<n; i++) {
-
+                v |= (long) (ram[address + i] & 0xFF) << (8 * i);
-    /**
+            }
-     * read {@code n} bytes from RAM
+        } else {
-     *
+            // TODO read from IO
-     * @param address address of first byte
-     * @param n number of bytes, max 8
-     * @return the unsigned bytes
-     */
-    long readRam(int address, int n) {
-        long v = 0;
-
-        for (int i=0; i<n; i++) {
-            v |= (long) (ram[address + i] & 0xFF) << (8 * i);
         }
 
         return v;
@@ -78,8 +68,14 @@ public class AddressSpace {
      * @param value the value
      */
     void writeRam(int address, int n, long value) {
-        for (int i=0; i<n; i++) {
+        if (address < romSize) {
-            ram[address + i] = (byte) (value >> (8 * i) & 0xFF);
+            throw new RuntimeException("Cannot write to Read Only Memory");
+        } else if (address < romSize + ramSize) {
+            for (int i=0; i<n; i++) {
+                ram[address + i] = (byte) (value >> (8 * i) & 0xFF);
+            }
+        } else {
+            // TODO write to IO
         }
     }
 }

src/main/java/eu/m724/CPU.java

@@ -4,31 +4,29 @@ package eu.m724;
  * Represents a rv32i CPU
  */
 public class CPU {
-    private final int xsize = 32;
     private final int[] register = new int[32];
 
     private AddressSpace addressSpace;
     private int programCounter;
 
     public void init(AddressSpace addressSpace) {
-        this.addressSpace = addressSpace; // TODO should addressspace really be linked
+        this.addressSpace = addressSpace; // TODO should addressspace really be here
 
         register[2] = addressSpace.romSize + addressSpace.ramSize;
         programCounter = addressSpace.romBase;
     }
 
-    public int getNextInstruction() {
+    private int getCurrentInstruction() {
         if (programCounter > addressSpace.romSize - 1)
             throw new NullPointerException();
 
-        int inst = (int) addressSpace.readRom(programCounter, 4);
+        return (int) addressSpace.read(programCounter, 4);
-        programCounter += 4;
-        return inst;
     }
 
     public boolean executeNextInstruction() {
         try {
-            executeInstruction(getNextInstruction());
+            executeInstruction(getCurrentInstruction());
+            programCounter += 4;
         } catch (NullPointerException e) {
             return false;
         }
@@ -42,6 +40,7 @@ public class CPU {
 
         if (opcode != 0) {
             System.out.println();
+            System.out.println("PC: " + programCounter);
             System.out.println("Instruction: " + Integer.toBinaryString(instruction));
             System.out.println("Opcode: " + Integer.toBinaryString(opcode));
             System.out.println("Destination register: " + Integer.toBinaryString(rd));
@@ -80,7 +79,10 @@ public class CPU {
         } else if (opcode == 0b0110111) { // LUI load upper immediate (U type)
 
         } else if (opcode == 0b0010111) { // AUIPC (add upper immediate to pc) (U type)
+            int imm = instruction & 0xFFFFF000;
+            register[rd] = imm + programCounter;
 
+            System.out.printf("AUIPC: pc %d + imm %d = %d -> x%d\n", programCounter, imm, register[rd], rd);
         } else if (opcode == 0b0110011) { // OP for Integer Register-Register Operations (R type)
             int funct3 = instruction >> 12 & 0x7;
 
@@ -93,7 +95,7 @@ public class CPU {
 
                     int sum = register[rs1] + register[rs2] & 0xFF; // to overflow
 
-                    System.out.printf("ADD: %d #%d + %d #%d = %d #%d\n", register[rs1], rs1, register[rs2], rs2, sum, rd);
+                    System.out.printf("ADD: %d x%d + %d x%d = %d x%d\n", register[rs1], rs1, register[rs2], rs2, sum, rd);
 
                     register[rd] = sum;
                 } else if (funct7 == 0b0100000) { // SUB
@@ -121,48 +123,131 @@ public class CPU {
 
             }
         } else if (opcode == 0b1101111) { // JAL for unconditional jump (J type)
+            System.out.println("JAL");
+            int imm = (instruction >> 31) << 20; // Extract imm[20] and sign-extend
+            imm |= (instruction >> 21) & 0x3FF; // Extract imm[10:1]
+            imm |= (instruction >> 20) & 0x1; // Extract imm[11]
+            imm |= (instruction >> 12) & 0xFF; // Extract imm[19:12]
+            imm <<= 1; // Left-shift by 1 to account for the implicit 0
 
+            register[rd] = programCounter; // program counter is always incremented after executing instruction
+            programCounter += imm;
+
+            System.out.printf("Jumped to %d + %d = %d (inst %d)\n", register[rd], imm, programCounter, programCounter / 4);
         } else if (opcode == 0b1100111) { // JALR for unconditional jump and link register (I type)
+            int funct3 = instruction >> 12 & 0x7;
 
+            if (funct3 != 0) {
+                // TODO throw an exception
+            }
+
+            int rs1 = instruction >> 15 & 0x1F;
+            int imm = instruction >> 20; // sign extends automatically
+
+            int result = register[rs1] + imm;
+            result &= ~1; // set LSB to 0
+
+            register[rd] = programCounter + 4;
+            programCounter = result - 4; // program counter is always incremented after executing instruction
+
+            System.out.printf("JALR: %d x%d + %d = %d (inst %d) -> x%d\n", register[rs1], rs1, imm, result, result / 4, rd);
         } else if (opcode == 0b1100011) { // BRANCH instruction (B type)
-            int funct3 = instruction >> 12 & 0x1F;
+            int funct3 = instruction >> 12 & 0x7;
+            int rs1 = instruction >> 15 & 0x1F;
+            int rs2 = instruction >> 20 & 0x1F;
+
+            int imm = (instruction >> 31) << 12; // imm[12] and sign extend
+            imm |= ((instruction >> 7) & 0x1) << 11;  // imm[11]
+            imm |= ((instruction >> 25) & 0x3f) << 5; // imm[10:5]
+            imm |= ((instruction >> 8) & 0xf) << 1;   // imm[4:1]
+            // imm[0] is implicitly 0
+
+            boolean branch = false;
 
             if (funct3 == 0b000) { // BEQ
-
+                System.out.printf("BEQ: #%d %d == #%d %d?\n", rs1, register[rs1], rs2, register[rs2]);
+                branch = register[rs1] == register[rs2];
             } else if (funct3 == 0b001) { // BNE
-
+                System.out.printf("BNE: #%d %d != #%d %d?\n", rs1, register[rs1], rs2, register[rs2]);
+                branch = register[rs1] != register[rs2];
             } else if (funct3 == 0b100) { // BLT
-
+                System.out.printf("BLT: #%d %d < #%d %d?\n", rs1, register[rs1], rs2, register[rs2]);
+                branch = register[rs1] < register[rs2];
             } else if (funct3 == 0b101) { // BGE
-
+                System.out.printf("BGE: #%d %d > #%d %d?\n", rs1, register[rs1], rs2, register[rs2]);
+                branch = register[rs1] > register[rs2];
             } else if (funct3 == 0b110) { // BLTU
-
+                System.out.printf("BLTU: #%d %d < #%d %d?\n", rs1, Integer.toUnsignedLong(register[rs1]), rs2, Integer.toUnsignedLong(register[rs2]));
+                branch = Integer.compareUnsigned(register[rs1], register[rs2]) < 0;
             } else if (funct3 == 0b111) { // BGEU
+                System.out.printf("BGEU: #%d %d > #%d %d?\n", rs1, Integer.toUnsignedLong(register[rs1]), rs2, Integer.toUnsignedLong(register[rs2]));
+                branch = Integer.compareUnsigned(register[rs1], register[rs2]) > 0;
+            } else {
+                // TODO illegal instruction
+            }
 
+            if (branch) {
+                programCounter += imm - 4; // program counter is always incremented after executing instruction so -4
             }
         } else if (opcode == 0b0000011) { // LOAD instruction (I type)
             int funct3 = instruction >> 12 & 0x1F;
+            int rs1 = instruction >> 15 & 0x1F;
+            int imm = instruction >> 20; // sign extends automatically
+            int addr = register[rs1] + imm;
 
             if (funct3 == 0b000) { // LB
+                // LB loads an 8-bit value from memory, then sign-extends to 32-bits.
+                register[rd] = (byte) addressSpace.read(addr, 1) >> 24;
 
+                System.out.printf("LB: addr: %d x%d + imm %d = %d | val: %d -> x%d\n", register[rs1], rs1, imm, addr, register[rd], rd);
             } else if (funct3 == 0b001) { // LH
+                // LH loads a 16-bit value from memory, then sign-extends to 32-bits.
+                register[rd] = (short) addressSpace.read(addr, 2) >> 16;
 
+                System.out.printf("LH: addr: %d x%d + imm %d = %d | val: %d -> x%d\n", register[rs1], rs1, imm, addr, register[rd], rd);
             } else if (funct3 == 0b010) { // LW
+                // LW instruction loads a 32-bit value from memory into rd.
+                register[rd] = (int) addressSpace.read(addr, 4);
 
+                System.out.printf("LW: addr: %d x%d + imm %d = %d | val: %d -> x%d\n", register[rs1], rs1, imm, addr, register[rd], rd);
             } else if (funct3 == 0b100) { // LBU
+                // LBU loads an 8-bit value from memory but then zero extends to 32-bits.
+                register[rd] = (int) addressSpace.read(addr, 1);
 
+                System.out.printf("LBU: addr: %d x%d + imm %d = %d | val: %d -> x%d\n", register[rs1], rs1, imm, addr, register[rd], rd);
             } else if (funct3 == 0b101) { // LHU
+                // LHU loads a 16-bit value from memory but then zero extends to 32-bits.
+                register[rd] = (int) addressSpace.read(addr, 2);
 
+                System.out.printf("LHU: addr: %d x%d + imm %d = %d | val: %d -> x%d\n", register[rs1], rs1, imm, addr, register[rd], rd);
+            } else {
+                // TODO illegal instruction
             }
         } else if (opcode == 0b0100011) { // STORE instruction (S type)
-            int funct3 = instruction >> 12 & 0x1F;
+            int funct3 = instruction >> 12 & 0x7;
+            int rs1 = instruction >> 15 & 0x1F;
+            int rs2 = instruction >> 20 & 0x1F;
+            int imm = instruction >> 20 & 0xffffffe0 | rd; // sign extends automatically
+            int addr = register[rs1] + imm;
+            System.out.println(addr);
 
             if (funct3 == 0b000) { // SB
+                // SB stores an 8-bit value from the low bits of register rs2 to memory.
+                addressSpace.writeRam(addr, 1, register[rs2] & 0xFF);
 
+                System.out.printf("SB: x%d %d -> %d\n", rs2, register[rs2] & 0xFF, addr);
             } else if (funct3 == 0b001) { // SH
+                // SH stores a 16-bit value from the low bits of register rs2 to memory.
+                addressSpace.writeRam(addr, 2, register[rs2] & 0xFFFF);
 
+                System.out.printf("SH: x%d %d -> %d\n", rs2, register[rs2] & 0xFFFF, addr);
             } else if (funct3 == 0b010) { // SW
+                // SW stores a 32-bit value from the low bits of register rs2 to memory.
+                addressSpace.writeRam(addr, 4, register[rs2]);
 
+                System.out.printf("SW: x%d %d -> %d\n", rs2, register[rs2], addr);
+            } else {
+                // TODO illegal instruction
             }
         } else if (opcode == 0b0001111) { // MISC-MEM for Memory Ordering Instructions (I type)
 

src/main/java/eu/m724/Main.java

@@ -3,17 +3,29 @@ package eu.m724;
 import java.util.Base64;
 
 public class Main {
-    public static void main(String[] args) {
+    public static void main(String[] args) throws InterruptedException {
-        AddressSpace addressSpace = new AddressSpace(1024, 1024); // 1 KiB
+        AddressSpace addressSpace = new AddressSpace(1024, 1024 * 1024); // 1 KiB
-        byte[] program = Base64.getDecoder().decode("kw5QABMPUAKzD98B");
+        //byte[] program = Base64.getDecoder().decode("kw5QABMPUAKzD98B"); // simple addition program
+        //byte[] program = Base64.getDecoder().decode("EwUQAGeAAAA="); // return value program
+        //byte[] program = Base64.getDecoder().decode("EwWgABcDAABnAAMAIyghASMmMQEjLhEAEwQFAJMEBQATCQAAkwkQAGP+mQAThfT/lwAAAOeAAACT" +
+        //        "hOT/MwmpAG/wn/6DIMEBE3UUAAMkgQGDJEEBgynBADMFJQEDKQEBEwEBAmeAAAA="); // 10th value of fibonacci
+        //byte[] program = Base64.getDecoder().decode("EwXwAhcDAABnAAMAIyghASMmMQEjLhEAEwQFAJMEBQATCQAAkwkQAGP+mQAThfT/lwAAAOeAAACT" +
+        //        "hOT/MwmpAG/wn/6DIMEBE3UUAAMkgQGDJEEBgynBADMFJQEDKQEBEwEBAmeAAAA="); // 47th value of fibonacci
+        //byte[] program = Base64.getDecoder().decode("EwUAAxcDAABnAAMAIyghASMmMQEjLhEAEwQFAJMEBQATCQAAkwkQAGP+mQAThfT/lwAAAOeAAACT" +
+        //        "hOT/MwmpAG/wn/6DIMEBE3UUAAMkgQGDJEEBgynBADMFJQEDKQEBEwEBAmeAAAA="); // 48th value of fibonacci (overflow)
+        byte[] program = Base64.getDecoder().decode("EwEB/iMuEQAjLIEAEwQBAiMmBP5vAAABgyfE/pOHFwAjJvT+AyfE/pMHkADj1uf+gyfE/hOFBwCD" +
+                "IMEBAySBARMBAQJngAAA"); // returns 10 after looping
         addressSpace.initRom(program);
 
         CPU cpu = new CPU();
         cpu.init(addressSpace);
+        cpu.setRegister(1, addressSpace.romSize); // make jumping to x1 end the program
 
-        while (cpu.executeNextInstruction()) { }
+        while (cpu.executeNextInstruction()) {
+            Thread.sleep(1000);
+        }
 
-        System.out.println("\nEnd of program");
+        System.out.println("\nEnd of program. Return value (x10): " + cpu.getRegister(10));
         cpu.dumpRegisters();
     }
 }

src/main/java/eu/m724/Memory.java

@@ -1,9 +0,0 @@
-package eu.m724;
-
-public class Memory {
-    private byte[] memory;
-
-    public Memory(int sizeBytes) {
-        memory = new byte[sizeBytes];
-    }
-}