diff --git a/tools/pioasm/pio_disassembler.cpp b/tools/pioasm/pio_disassembler.cpp
new file mode 100644
index 0000000..c30fb0b
--- /dev/null
+++ b/tools/pioasm/pio_disassembler.cpp
@@ -0,0 +1,171 @@
+/*
+ * Copyright (c) 2020 Raspberry Pi (Trading) Ltd.
+ *
+ * SPDX-License-Identifier: BSD-3-Clause
+ */
+
+#include <array>
+#include <sstream>
+#include <iomanip>
+#include "pio_disassembler.h"
+
+extern "C" void disassemble(char *buf, int buf_len, uint16_t inst, uint sideset_bits, bool sideset_opt) {
+    if (buf_len) buf[disassemble(inst, sideset_bits, sideset_opt).copy(buf, buf_len - 1)] = 0;
+}
+
+std::string disassemble(uint16_t inst, uint sideset_bits_including_opt, bool sideset_opt) {
+    std::stringstream ss;
+    uint major = inst >> 13u;
+    uint arg1 = ((uint) inst >> 5u) & 0x7u;
+    uint arg2 = inst & 0x1fu;
+    auto op = [&](const std::string &s) {
+        ss << std::left << std::setw(7) << s;
+    };
+    auto op_guts = [&](const std::string &s) {
+        ss << std::left << std::setw(16) << s;
+    };
+
+    bool invalid = false;
+    switch (major) {
+        case 0b000: {
+            static std::array<std::string, 8> conditions{"", "!x, ", "x--, ", "!y, ", "y--, ", "x != y, ", "pin, ",
+                                                         "!osre, "};
+            op("jmp");
+            op_guts(conditions[arg1] + std::to_string(arg2));
+            break;
+        }
+        case 0b001: {
+            uint source = arg1 & 3u;
+            std::string guts;
+            switch (source) {
+                case 0b00:
+                    guts = "gpio, " + std::to_string(arg2);
+                    break;
+                case 0b01:
+                    guts = "pin, " + std::to_string(arg2);
+                    break;
+                case 0b10:
+                    if (arg2 & 0x8u) {
+                        invalid = true;
+                    } else {
+                        guts = "irq, " + std::to_string(arg2 & 7u);
+                        if (arg2 & 0x10u) {
+                            guts += " rel";
+                        }
+                    }
+                    break;
+            }
+            if (!invalid) {
+                guts = ((arg1 & 4u) ? "1 " : "0 ") + guts;
+                op("wait");
+                op_guts(guts);
+            }
+            break;
+        }
+        case 0b010: {
+            static std::array<std::string, 8> sources { "pins", "x", "y", "null", "", "status", "isr", "osr"};
+            std::string source = sources[arg1];
+            if (source.empty()) {
+                invalid = true;
+            } else {
+                op("in");
+                op_guts(source + ", " + std::to_string(arg2 ? arg2 : 32));
+            }
+            break;
+        }
+        case 0b011: {
+            static std::array<std::string, 8> dests { "pins", "x", "y", "null", "pindirs", "pc", "isr", "exec"};
+            op("out");
+            op_guts(dests[arg1] + ", " + std::to_string(arg2 ? arg2 : 32));
+            break;
+        }
+        case 0b100: {
+            if (arg2) {
+                invalid = true;
+            } else {
+                std::string guts = "";
+                if (arg1 & 4u) {
+                    op("pull");
+                    if (arg1 & 2u) guts = "ifempty ";
+                } else {
+                    op("push");
+                    if (arg1 & 2u) guts = "iffull ";
+                }
+                guts += (arg1 & 0x1u) ? "block" : "noblock";
+                op_guts(guts);
+            }
+            break;
+        }
+        case 0b101: {
+            static std::array<std::string, 8> dests { "pins", "x", "y", "", "exec", "pc", "isr", "osr"};
+            static std::array<std::string, 8> sources { "pins", "x", "y", "null", "", "status", "isr", "osr"};
+            std::string dest = dests[arg1];
+            std::string source = sources[arg2 & 7u];
+            uint operation = arg2 >> 3u;
+            if (source.empty() || dest.empty() || operation == 3) {
+                invalid = true;
+            }
+            if (dest == source && !operation && (arg1 == 1 || arg2 == 2)) {
+                op("nop");
+                op_guts("");
+            } else {
+                op("mov");
+                std::string guts = dest + ", ";
+                if (operation == 1) {
+                    guts += "!";
+                } else if (operation == 2) {
+                    guts += "::";
+                }
+                guts += source;
+                op_guts(guts);
+            }
+            break;
+        }
+        case 0b110: {
+            if ((arg1 & 0x4u) || (arg2 & 0x8u)) {
+                invalid = true;
+            } else {
+                op("irq");
+                std::string guts;
+                if (arg1 & 0x2u) {
+                    guts += "clear ";
+                } else if (arg1 & 0x1u) {
+                    guts += "wait ";
+                } else {
+                    guts += "nowait ";
+                }
+                guts += std::to_string(arg2 & 7u);
+                if (arg2 & 0x10u) {
+                    guts += " rel";
+                }
+                op_guts(guts);
+            }
+            break;
+        }
+        case 0b111: {
+            static std::array<std::string, 8> dests{"pins", "x", "y", "", "pindirs", "", "", ""};
+            std::string dest = dests[arg1];
+            if (dest.empty()) {
+                invalid = true;
+            } else {
+                op("set");
+                op_guts(dests[arg1] + ", " + std::to_string(arg2));
+            }
+            break;
+        }
+    }
+    if (invalid) {
+        return "reserved";
+    }
+    uint delay = ((uint) inst >> 8u) & 0x1f;
+    ss << std::left << std::setw(7);
+    if (sideset_bits_including_opt && (!sideset_opt || (delay & 0x10u))) {
+        ss << ("side "+ std::to_string((delay & (sideset_opt ? 0xfu : 0x1fu)) >> (5u - sideset_bits_including_opt)));
+    } else {
+        ss << "";
+    }
+    delay &= ((1u << (5 - sideset_bits_including_opt)) - 1u);
+    ss << std::left << std::setw(4) << (delay ? ("[" + std::to_string(delay) + "]") : "");
+    return ss.str();
+}
+