README: Add usage example

README.md

@@ -37,3 +37,124 @@ Plans
 * ARMv7A guest support
 * ARMv5 guest support
 * ARMv8 host support
+
+Usage Example
+-------------
+
+The below is a minimal example. Bring-your-own memory system.
+
+```cpp
+#include <array>
+#include <cstdint>
+#include <cstdio>
+#include <exception>
+
+#include <dynarmic/A32/a32.h>
+#include <dynarmic/A32/config.h>
+
+using u8 = std::uint8_t;
+using u16 = std::uint16_t;
+using u32 = std::uint32_t;
+using u64 = std::uint64_t;
+
+class MyEnvironment final : public Dynarmic::A32::UserCallbacks {
+public:
+    u64 ticks_left = 0;
+    std::array<u8, 2048> memory{};
+
+    u8 MemoryRead8(u32 vaddr) override {
+        if (vaddr >= memory.size()) {
+            return 0;
+        }
+        return memory[vaddr];
+    }
+
+    u16 MemoryRead16(u32 vaddr) override {
+        return u16(MemoryRead8(vaddr)) | u16(MemoryRead8(vaddr + 1)) << 8;
+    }
+
+    u32 MemoryRead32(u32 vaddr) override {
+        return u32(MemoryRead16(vaddr)) | u32(MemoryRead16(vaddr + 2)) << 16;
+    }
+
+    u64 MemoryRead64(u32 vaddr) override {
+        return u64(MemoryRead32(vaddr)) | u64(MemoryRead32(vaddr + 4)) << 32;
+    }
+
+    void MemoryWrite8(u32 vaddr, u8 value) override {
+        if (vaddr >= memory.size()) {
+            return;
+        }
+        memory[vaddr] = value;
+    }
+
+    void MemoryWrite16(u32 vaddr, u16 value) override {
+        MemoryWrite8(vaddr, u8(value));
+        MemoryWrite8(vaddr + 1, u8(value >> 8));
+    }
+
+    void MemoryWrite32(u32 vaddr, u32 value) override {
+        MemoryWrite16(vaddr, u16(value));
+        MemoryWrite16(vaddr + 2, u16(value >> 16));
+    }
+
+    void MemoryWrite64(u32 vaddr, u64 value) override {
+        MemoryWrite32(vaddr, u32(value));
+        MemoryWrite32(vaddr + 4, u32(value >> 32));
+    }
+
+    void InterpreterFallback(u32 pc, size_t num_instructions) override {
+        // This is never called in practice.
+        std::terminate();
+    }
+
+    void CallSVC(u32 swi) override {
+        // Do something.
+    }
+
+    void ExceptionRaised(u32 pc, Dynarmic::A32::Exception exception) override {
+        // Do something.
+    }
+
+    void AddTicks(u64 ticks) override {
+        if (ticks > ticks_left) {
+            ticks_left = 0;
+            return;
+        }
+        ticks_left -= ticks;
+    }
+
+    u64 GetTicksRemaining() override {
+        return ticks_left;
+    }
+};
+
+int main(int argc, char** argv) {
+    MyEnvironment env;
+    Dynarmic::A32::UserConfig user_config;
+    user_config.callbacks = &env;
+    Dynarmic::A32::Jit cpu{user_config};
+
+    // Execute at least 1 instruction.
+    // (Note: More than one instruction may be executed.)
+    env.ticks_left = 1;
+
+    // Write some code to memory.
+    env.MemoryWrite16(0, 0x0088); // lsls r0, r1, #2
+    env.MemoryWrite16(2, 0xE7FE); // b +#0 (infinite loop)
+
+    // Setup registers.
+    cpu.Regs()[0] = 1;
+    cpu.Regs()[1] = 2;
+    cpu.Regs()[15] = 0; // PC = 0
+    cpu.SetCpsr(0x00000030); // Thumb mode
+
+    // Execute!
+    cpu.Run();
+
+    // Here we would expect jit.Regs()[0] == 8
+    printf("R0: %u\n", jit.Regs()[0]);
+
+    return 0;
+}
+```