From 616f70bfa48619b75995714ff454d962ab57ea41 Mon Sep 17 00:00:00 2001
From: Konstantin Nazarov <mail@knazarov.com>
Date: Sun, 22 Dec 2024 14:07:57 +0000
Subject: [PATCH] Implement UART closer to how it's done on the sifive board

---
 src/vm.cpp | 20 ++++++++++++++++----
 src/vm.hpp |  6 +++---
 2 files changed, 19 insertions(+), 7 deletions(-)

diff --git a/src/vm.cpp b/src/vm.cpp
index 5644d1d..78c8e44 100644
--- a/src/vm.cpp
+++ b/src/vm.cpp
@@ -37,9 +37,13 @@ void UART::read_mem_u8(uint8_t* dst, size_t addr) {
   }
   addr -= UART_ADDR;
   switch (addr) {
-    case UART_LSR:
-      // Always ready to transmit
-      *dst = LSR_TRANSMITTER_EMPTY;
+    case UART_RXDATA:
+      *dst = 0x0;
+      break;
+    case UART_RXDATA + 3:
+      // receiver is always empty for now
+      *dst = 0x80;
+      break;
     default:
       *dst = 0;
   }
@@ -52,7 +56,7 @@ void UART::write_mem_u8(uint8_t* src, size_t addr) {
   }
   addr -= UART_ADDR;
   switch (addr) {
-    case UART_THR:
+    case UART_TXDATA:
       std::cout.put(static_cast<char>(*src));
       break;
   }
@@ -66,6 +70,14 @@ void UART::write_mem_u32(uint32_t* src, size_t addr) {
   }
 }
 
+void UART::read_mem_u32(uint32_t* dst, size_t addr) {
+  uint8_t* d = (uint8_t*)dst;
+
+  for (size_t i = 0; i < 4; i++) {
+    read_mem_u8(d + i, addr + i);
+  }
+}
+
 void Timer::update() {
   using Clock = std::chrono::high_resolution_clock;
   constexpr auto den = Clock::period::den;
diff --git a/src/vm.hpp b/src/vm.hpp
index a9c3458..824c99f 100644
--- a/src/vm.hpp
+++ b/src/vm.hpp
@@ -78,12 +78,12 @@ class UART final : public Device {
   virtual void write_mem_u32(uint32_t *src, size_t addr);
 
   virtual void read_mem_u8(uint8_t *dst, size_t addr);
+  virtual void read_mem_u32(uint32_t *dst, size_t addr);
 
  private:
   enum Registers {
-    UART_RBR = 0x00,  // Receiver Buffer Register
-    UART_THR = 0x00,  // Transmitter Holding Register
-    UART_LSR = 0x05   // Line Status Register
+    UART_TXDATA = 0x00,
+    UART_RXDATA = 0x04,
   };
 
   // Line Status Register bits