1 files changed, 80 insertions, 0 deletions
diff --git a/src/uart.zig b/src/uart.zig
new file mode 100644
index 0000000..661084c
--- /dev/null
+++ b/src/uart.zig
@@ -0,0 +1,80 @@
+const std = @import("std");
+
+var uart_ptr: ?[*]u8 = null;
+
+pub fn init(base_addr: usize) void {
+    uart_ptr = @ptrFromInt(base_addr);
+    // Set the word length to 8 bits. Offset 3 is the LCR (line control
+    // register).
+    //
+    // The bottom 2 bits set the word length:
+    // 0 = 5 bits
+    // 1 = 6 bits
+    // 2 = 7 bits
+    // 3 = 8 bits
+    const lcr: u8 = 3 << 0;
+    uart_ptr.?[3] = lcr;
+    // Now enable the FIFO, which is the bottom bit of the FIFO control
+    // register (offset 2)
+    uart_ptr.?[2] = 1 << 0;
+    // Enable receiver buffer interrupts, which is the bottom bit of the IER
+    // (interrupt enable register)
+    uart_ptr.?[1] = 1 << 0;
+    // Per the datasheet, we are to set the clock divisor to:
+    // divisor = ceil(clock_hz/(baud_sps * 16))
+    // divisor = ceil(22_729_000/(115200 * 16)) = 13
+    const divisor: u16 = 13;
+    const divisor_least: u8 = divisor & 0xff;
+    const divisor_most: u8 = divisor >> 8;
+    // To write the divisor, we have to open the divisor latch on bit 7 of the
+    // line control register.
+    uart_ptr.?[3] = lcr | 1 << 7;
+    // Now we write the divisor
+    uart_ptr.?[0] = divisor_least;
+    uart_ptr.?[1] = divisor_most;
+    // Now we lock it again
+    uart_ptr.?[3] = lcr;
+}
+
+pub fn get() ?u8 {
+    if (uart_ptr.?[5] & 1 != 0) {
+        return uart_ptr.?[0];
+    } else {
+        return null;
+    }
+}
+
+pub fn put(c: u8) void {
+    uart_ptr.?[0] = c;
+}
+
+fn drain(_: *std.Io.Writer, data: []const []const u8, splat: usize) !usize {
+    if (uart_ptr == null) return std.Io.Writer.Error.WriteFailed;
+    var written: u32 = 0;
+    for (data, 0..) |item, i| {
+        if (i == data.len - 1) {
+            for (0..splat) |_| for (item) |c| {
+                put(c);
+                written += 1;
+            };
+        } else {
+            for (item) |c| {
+                put(c);
+                written += 1;
+            }
+        }
+    }
+    return written;
+}
+
+pub var writer: std.Io.Writer = .{
+    .buffer = &.{},
+    .vtable = &.{
+        .drain = drain,
+    },
+};
+
+pub var terminal: std.Io.Terminal = .{
+    .writer = &writer,
+    .mode = .escape_codes,
+};