From 0b5ccf48b8ea9804016d77fe072820ac7ccb7c33 Mon Sep 17 00:00:00 2001
From: Byron Lathi <bslathi19@gmail.com>
Date: Mon, 14 Mar 2022 00:24:08 -0500
Subject: [PATCH] Add basic UART driver

This didn't have a chance to get tested so I hope it works.

There is not fancy stuff here, just write to the register and wait till
it is done.
---
 sw/io.inc65 |  7 ++++++-
 sw/uart.h   | 11 +++++++++++
 sw/uart.s   | 29 +++++++++++++++++++++++++++++
 3 files changed, 46 insertions(+), 1 deletion(-)
 create mode 100644 sw/uart.h
 create mode 100644 sw/uart.s

diff --git a/sw/io.inc65 b/sw/io.inc65
index 4bf4736..9192cff 100644
--- a/sw/io.inc65
+++ b/sw/io.inc65
@@ -1 +1,6 @@
-SEVEN_SEG  = $7ff0
+SEVEN_SEG   = $7ff0
+
+UART        = $7ff4
+UART_TXB    = UART
+UART_RXB    = UART
+UART_STATUS = UART + 1
diff --git a/sw/uart.h b/sw/uart.h
new file mode 100644
index 0000000..c96b48c
--- /dev/null
+++ b/sw/uart.h
@@ -0,0 +1,11 @@
+#ifndef _UART_H
+#define _UART_H
+
+#include <stdint.h>
+
+void uart_txb(uint8_t val);
+void uart_txb_block(uint8_t val);
+
+uint8_t uart_status();
+
+#endif
\ No newline at end of file
diff --git a/sw/uart.s b/sw/uart.s
new file mode 100644
index 0000000..80b5563
--- /dev/null
+++ b/sw/uart.s
@@ -0,0 +1,29 @@
+.include "io.inc65"
+
+.importzp sp, sreg
+
+.export _uart_txb
+.export _uart_txb_block
+.export _uart_status
+
+.autoimport	on
+
+.code
+
+; @in A: byte to transmit
+; Transmits a byte over the UART
+_uart_txb:
+        sta UART_TXB        ; Just write value, don't wait
+        rts
+
+_uart_txb_block:
+        sta UART_TXB        ; Write value
+@1:     lda UART_STATUS     ; Wait for status[0] to be 0
+        bit #$01
+        bne @1
+        rts
+
+_uart_status:
+        lda UART_STATUS
+        ldx #$00
+        rts