From 82884b3ef726889f837542d2b5e98e2cd8246c00 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Karel=20Ko=C4=8D=C3=AD?= <cynerd@email.cz>
Date: Thu, 31 Aug 2017 00:18:53 +0200
Subject: Handle endianness in memory correctly

---
 qtmips_machine/memory.cpp           | 29 ++++++++++++++++++++++++++++-
 qtmips_machine/programmemory.cpp    |  1 -
 qtmips_machine/tests/testmemory.cpp | 22 ++++++++++++++++++++++
 qtmips_machine/tests/tst_machine.h  |  1 +
 4 files changed, 51 insertions(+), 2 deletions(-)

diff --git a/qtmips_machine/memory.cpp b/qtmips_machine/memory.cpp
index 25deab0..2540035 100644
--- a/qtmips_machine/memory.cpp
+++ b/qtmips_machine/memory.cpp
@@ -1,30 +1,57 @@
 #include "memory.h"
 
+// Note about endianness: Current memory implementation is expected to be a big endian.
+// But we can be running on little endian so we should do conversion from bytes to word according that.
+
 void MemoryAccess::write_hword(std::uint32_t offset, std::uint16_t value) {
+#if __BYTE_ORDER == __LITTLE_ENDIAN
+    this->write_byte(offset++, (std::uint8_t)(value >> 8));
+    this->write_byte(offset, (std::uint8_t)value);
+#else
     this->write_byte(offset++, (std::uint8_t)value);
     this->write_byte(offset, (std::uint8_t)(value >> 8));
+#endif
 }
 
 void MemoryAccess::write_word(std::uint32_t offset, std::uint32_t value) {
+#if __BYTE_ORDER == __LITTLE_ENDIAN
+    this->write_byte(offset++, (std::uint8_t)(value >> 24));
+    this->write_byte(offset++, (std::uint8_t)(value >> 16));
+    this->write_byte(offset++, (std::uint8_t)(value >> 8));
+    this->write_byte(offset, (std::uint8_t)value);
+#else
     this->write_byte(offset++, (std::uint8_t)value);
     this->write_byte(offset++, (std::uint8_t)(value >> 8));
     this->write_byte(offset++, (std::uint8_t)(value >> 16));
-    this->write_byte(offset++, (std::uint8_t)(value >> 24));
+    this->write_byte(offset, (std::uint8_t)(value >> 24));
+#endif
 }
 
 std::uint16_t MemoryAccess::read_hword(std::uint32_t offset) {
     std::uint16_t dt = 0;
+#if __BYTE_ORDER == __LITTLE_ENDIAN
+    dt |= (this->read_byte(offset++) << 8);
+    dt |= this->read_byte(offset);
+#else
     dt |= this->read_byte(offset++);
     dt |= (this->read_byte(offset) << 8);
+#endif
     return dt;
 }
 
 std::uint32_t MemoryAccess::read_word(std::uint32_t offset) {
     std::uint32_t dt = 0;
+#if __BYTE_ORDER == __LITTLE_ENDIAN
+    dt |= (this->read_byte(offset++) << 24);
+    dt |= (this->read_byte(offset++) << 16);
+    dt |= (this->read_byte(offset++) << 8);
+    dt |= this->read_byte(offset);
+#else
     dt |= this->read_byte(offset++);
     dt |= (this->read_byte(offset++) << 8);
     dt |= (this->read_byte(offset++) << 16);
     dt |= (this->read_byte(offset) << 24);
+#endif
     return dt;
 }
 
diff --git a/qtmips_machine/programmemory.cpp b/qtmips_machine/programmemory.cpp
index 6759842..1ff3bca 100644
--- a/qtmips_machine/programmemory.cpp
+++ b/qtmips_machine/programmemory.cpp
@@ -40,7 +40,6 @@ Instruction *ProgramMemory::at(std::uint32_t address) {
     }
 }
 
-// TODO implement
 #define I_UNKNOWN(DATA) throw QTMIPS_EXCEPTION(UnsupportedInstruction, "Unknown instruction, can't decode", to_string_hex(DATA))
 #define I_UNSUPPORTED(INST) throw QTMIPS_EXCEPTION(UnsupportedInstruction, "Decoded unsupported unstruction", #INST)
 
diff --git a/qtmips_machine/tests/testmemory.cpp b/qtmips_machine/tests/testmemory.cpp
index 991e1f1..eac7dd6 100644
--- a/qtmips_machine/tests/testmemory.cpp
+++ b/qtmips_machine/tests/testmemory.cpp
@@ -59,3 +59,25 @@ void MachineTests::memory_section() {
    // Read trough memory
    QCOMPARE(m.read_byte(address), (std::uint8_t)0x66);
 }
+
+void MachineTests::memory_endian() {
+    Memory m;
+
+    // Memory should be bit endian so write bytes from most significant byte
+    m.write_byte(0x00, 0x12);
+    m.write_byte(0x01, 0x34);
+    m.write_byte(0x02, 0x56);
+    m.write_byte(0x03, 0x78);
+    QCOMPARE(m.read_hword(0x00), (std::uint16_t)0x1234);
+    QCOMPARE(m.read_word(0x00), (std::uint32_t)0x12345678);
+
+    m.write_hword(0x80, 0x1234);
+    QCOMPARE(m.read_byte(0x80), (std::uint8_t)0x12);
+    QCOMPARE(m.read_byte(0x81), (std::uint8_t)0x34);
+
+    m.write_word(0xF0, 0x12345678);
+    QCOMPARE(m.read_byte(0xF0), (std::uint8_t)0x12);
+    QCOMPARE(m.read_byte(0xF1), (std::uint8_t)0x34);
+    QCOMPARE(m.read_byte(0xF2), (std::uint8_t)0x56);
+    QCOMPARE(m.read_byte(0xF3), (std::uint8_t)0x78);
+}
diff --git a/qtmips_machine/tests/tst_machine.h b/qtmips_machine/tests/tst_machine.h
index 214ab88..e5137c4 100644
--- a/qtmips_machine/tests/tst_machine.h
+++ b/qtmips_machine/tests/tst_machine.h
@@ -16,6 +16,7 @@ private Q_SLOTS:
     void memory_data();
     void memory_section();
     void memory_section_data();
+    void memory_endian();
 };
 
 #endif // TST_MACHINE_H
-- 
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