From 62302e03d56cfd93c7fbda8a9f42a068d1725660 Mon Sep 17 00:00:00 2001
From: xamidev <xamidev@riseup.net>
Date: Sun, 21 Dec 2025 20:33:48 +0100
Subject: [PATCH] Add: init serial + getting text out of it

---
 Makefile        |  4 ++--
 README.md       |  6 ++++-
 src/io/serial.c | 59 +++++++++++++++++++++++++++++++++++++++++++++++++
 src/io/serial.h | 10 +++++++++
 src/kernel.h    |  3 ++-
 src/kmain.c     |  5 +++++
 6 files changed, 83 insertions(+), 4 deletions(-)
 create mode 100644 src/io/serial.c
 create mode 100644 src/io/serial.h

diff --git a/Makefile b/Makefile
index a0f315c..c26c999 100644
--- a/Makefile
+++ b/Makefile
@@ -1,6 +1,6 @@
 build:
 	rm -f *.o
-	x86_64-elf-gcc -g -c -I src src/io/term.c src/io/printf.c src/kmain.c -Wall -Wextra -std=gnu99 -nostdlib -ffreestanding -fno-stack-protector -fno-stack-check -fno-PIC -ffunction-sections -fdata-sections -mcmodel=kernel
+	x86_64-elf-gcc -g -c -I src src/io/serial.c src/io/term.c src/io/printf.c src/kmain.c -Wall -Wextra -std=gnu99 -nostdlib -ffreestanding -fno-stack-protector -fno-stack-check -fno-PIC -ffunction-sections -fdata-sections -mcmodel=kernel
 	objcopy -O elf64-x86-64 -B i386 -I binary zap-light16.psf zap-light16.o
 	x86_64-elf-ld -o pepperk -T linker.ld *.o
 
@@ -31,7 +31,7 @@ debug:
 	gdb pepperk --command=debug.gdb
 
 run: build-iso
-	qemu-system-x86_64 -cdrom pepper.iso
+	qemu-system-x86_64 -cdrom pepper.iso -serial stdio
 
 clean:
 	rm -rf *.o pepperk iso_root pepper.iso limine
diff --git a/README.md b/README.md
index 60fb593..9ce9a07 100644
--- a/README.md
+++ b/README.md
@@ -13,4 +13,8 @@ PepperOS wouldn't be possible without the following freely-licensed software:
 
 - the [Limine](https://codeberg.org/Limine/Limine) portable bootloader
 - Marco Paland's freestanding [printf implementation](https://github.com/mpaland)
-- the [ZAP](https://www.zap.org.au/projects/console-fonts-zap/) PSF console fonts
\ No newline at end of file
+- the [ZAP](https://www.zap.org.au/projects/console-fonts-zap/) PSF console fonts
+
+...and without these amazing resources:
+
+- the [OSDev](https://osdev.org) wiki & forums
\ No newline at end of file
diff --git a/src/io/serial.c b/src/io/serial.c
new file mode 100644
index 0000000..7584dfa
--- /dev/null
+++ b/src/io/serial.c
@@ -0,0 +1,59 @@
+#include "../kernel.h"
+
+void outb(int port, unsigned char data)
+{
+    __asm__ __volatile__("outb %%al, %%dx" :: "a" (data),"d" (port));
+}
+
+unsigned char inb(int port)
+{
+    unsigned char data = 0;
+    __asm__ __volatile__("inb %%dx, %%al" : "=a" (data) : "d" (port));
+    return data;
+}
+
+// COM1
+#define PORT 0x3F8
+
+int serial_init()
+{
+    outb(PORT + 1, 0x00);      // Disable all interrupts
+    outb(PORT + 3, 0x80);    // Enable DLAB (set baud rate divisor)
+    outb(PORT + 0, 0x03);    // Set divisor to 3 (lo byte) 38400 baud
+    outb(PORT + 1, 0x00);    //                  (hi byte)
+    outb(PORT + 3, 0x03);    // 8 bits, no parity, one stop bit
+    outb(PORT + 2, 0xC7);    // Enable FIFO, clear them, with 14-byte threshold
+    outb(PORT + 4, 0x0B);    // IRQs enabled, RTS/DSR set
+    outb(PORT + 4, 0x1E);    // Set in loopback mode, test the serial chip
+    outb(PORT + 0, 0xAE);    // Test serial chip (send byte 0xAE and check if serial returns same byte)
+
+    if (inb(PORT) != 0xAE)
+    {
+        return -EIO;
+    }
+
+    // Set normal operation mode
+    outb(PORT + 4, 0x0F);
+    return 0;
+}
+
+static int is_transmit_empty()
+{
+    return inb(PORT + 5) & 0x20;
+}
+
+void write_serial(char c)
+{
+    while (!is_transmit_empty()); // wait for free spot
+    outb(PORT, c);
+}
+
+void serial_kputs(const char* str)
+{
+    unsigned int i=0;
+    while (str[i])
+    {
+        write_serial(str[i]);
+        i++;
+    }
+}
\ No newline at end of file
diff --git a/src/io/serial.h b/src/io/serial.h
new file mode 100644
index 0000000..698fc0d
--- /dev/null
+++ b/src/io/serial.h
@@ -0,0 +1,10 @@
+#ifndef SERIAL_H
+#define SERIAL_H
+
+void outb(int port, unsigned char data);
+unsigned char inb(int port);
+
+int serial_init();
+void serial_kputs(const char* str);
+
+#endif
\ No newline at end of file
diff --git a/src/kernel.h b/src/kernel.h
index 918fa6a..c3769ea 100644
--- a/src/kernel.h
+++ b/src/kernel.h
@@ -3,7 +3,8 @@
 
 enum ErrorCodes
 {
-	ENOMEM
+	ENOMEM,
+	EIO
 };
 
 #endif
diff --git a/src/kmain.c b/src/kmain.c
index d818962..b570eca 100644
--- a/src/kmain.c
+++ b/src/kmain.c
@@ -3,6 +3,7 @@
 #include <limine.h>
 #include "io/term.h"
 #include "io/printf.h"
+#include "io/serial.h"
 
 // Limine version used
 __attribute__((used, section(".limine_requests")))
@@ -111,6 +112,10 @@ void kmain()
 
 	if (term_init()) hcf();
 
+	if (serial_init()) kputs("kernel: serial: error: Cannot init serial communication!");
+	
+	serial_kputs("\n\nkernel: serial: Hello, world from serial!");
+
 	// Draw something
 	printf("%s, %s!", "Hello", "world");
 
-- 
2.49.1