From 4e7666742e6089e7f0ff359c42f726acde7e2a0c Mon Sep 17 00:00:00 2001
From: Wojtek Kosior <kwojtus@protonmail.com>
Date: Thu, 3 Oct 2019 11:00:21 +0200
Subject: copy-paste broken code from wiki.osdev; fix it; add Makefile

---
 kernel.c | 143 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 143 insertions(+)
 create mode 100644 kernel.c

(limited to 'kernel.c')

diff --git a/kernel.c b/kernel.c
new file mode 100644
index 0000000..6d09ce7
--- /dev/null
+++ b/kernel.c
@@ -0,0 +1,143 @@
+#include <stddef.h>
+#include <stdint.h>
+ 
+// board type, raspi2
+#define RASPI 2
+ 
+// Memory-Mapped I/O output
+static inline void mmio_write(uint32_t reg, uint32_t data)
+{
+  *(volatile uint32_t*)reg = data;
+}
+ 
+// Memory-Mapped I/O input
+static inline uint32_t mmio_read(uint32_t reg)
+{
+  return *(volatile uint32_t*)reg;
+}
+ 
+// Loop <delay> times in a way that the compiler won't optimize away
+static inline void delay(int32_t count)
+{
+  asm volatile("__delay_%=: subs %[count], %[count], #1; bne __delay_%=\n"
+	       : "=r"(count): [count]"0"(count) : "cc");
+}
+
+#if RASPI == 4
+#define GPIO_BASE 0xFE200000
+#else
+#if RASPI == 3 || RASPI == 2
+#define GPIO_BASE 0x3F200000
+#else
+#define GPIO_BASE 0x20200000
+#endif // RASPI == 3 || RASPI == 2
+#endif // RASPI == 4
+
+enum
+  {
+    // The offsets for reach register.
+ 
+    // Controls actuation of pull up/down to ALL GPIO pins.
+    GPPUD = (GPIO_BASE + 0x94),
+ 
+    // Controls actuation of pull up/down for specific GPIO pin.
+    GPPUDCLK0 = (GPIO_BASE + 0x98),
+ 
+    // The base address for UART.
+    UART0_BASE = 0x3F201000, // for raspi2 & 3, 0x20201000 for raspi1
+ 
+    // The offsets for reach register for the UART.
+    UART0_DR     = (UART0_BASE + 0x00),
+    UART0_RSRECR = (UART0_BASE + 0x04),
+    UART0_FR     = (UART0_BASE + 0x18),
+    UART0_ILPR   = (UART0_BASE + 0x20),
+    UART0_IBRD   = (UART0_BASE + 0x24),
+    UART0_FBRD   = (UART0_BASE + 0x28),
+    UART0_LCRH   = (UART0_BASE + 0x2C),
+    UART0_CR     = (UART0_BASE + 0x30),
+    UART0_IFLS   = (UART0_BASE + 0x34),
+    UART0_IMSC   = (UART0_BASE + 0x38),
+    UART0_RIS    = (UART0_BASE + 0x3C),
+    UART0_MIS    = (UART0_BASE + 0x40),
+    UART0_ICR    = (UART0_BASE + 0x44),
+    UART0_DMACR  = (UART0_BASE + 0x48),
+    UART0_ITCR   = (UART0_BASE + 0x80),
+    UART0_ITIP   = (UART0_BASE + 0x84),
+    UART0_ITOP   = (UART0_BASE + 0x88),
+    UART0_TDR    = (UART0_BASE + 0x8C),
+  };
+ 
+void uart_init()
+{
+  // Disable UART0.
+  mmio_write(UART0_CR, 0x00000000);
+  // Setup the GPIO pin 14 && 15.
+ 
+  // Disable pull up/down for all GPIO pins & delay for 150 cycles.
+  mmio_write(GPPUD, 0x00000000);
+  delay(150);
+ 
+  // Disable pull up/down for pin 14,15 & delay for 150 cycles.
+  mmio_write(GPPUDCLK0, (1 << 14) | (1 << 15));
+  delay(150);
+ 
+  // Write 0 to GPPUDCLK0 to make it take effect.
+  mmio_write(GPPUDCLK0, 0x00000000);
+ 
+  // Clear pending interrupts.
+  mmio_write(UART0_ICR, 0x7FF);
+ 
+  // Set integer & fractional part of baud rate.
+  // Divider = UART_CLOCK/(16 * Baud)
+  // Fraction part register = (Fractional part * 64) + 0.5
+  // UART_CLOCK = 3000000; Baud = 115200.
+ 
+  // Divider = 3000000 / (16 * 115200) = 1.627 = ~1.
+  mmio_write(UART0_IBRD, 1);
+  // Fractional part register = (.627 * 64) + 0.5 = 40.6 = ~40.
+  mmio_write(UART0_FBRD, 40);
+ 
+  // Enable FIFO & 8 bit data transmission (1 stop bit, no parity).
+  mmio_write(UART0_LCRH, (1 << 4) | (1 << 5) | (1 << 6));
+ 
+  // Mask all interrupts.
+  mmio_write(UART0_IMSC, (1 << 1) | (1 << 4) | (1 << 5) | (1 << 6) |
+	     (1 << 7) | (1 << 8) | (1 << 9) | (1 << 10));
+ 
+  // Enable UART0, receive & transfer part of UART.
+  mmio_write(UART0_CR, (1 << 0) | (1 << 8) | (1 << 9));
+}
+ 
+void uart_putc(unsigned char c)
+{
+  // Wait for UART to become ready to transmit.
+  while ( mmio_read(UART0_FR) & (1 << 5) ) { }
+  mmio_write(UART0_DR, c);
+}
+ 
+unsigned char uart_getc()
+{
+  // Wait for UART to have received something.
+  while ( mmio_read(UART0_FR) & (1 << 4) ) { }
+  return mmio_read(UART0_DR);
+}
+ 
+void uart_puts(const char* str)
+{
+  for (size_t i = 0; str[i] != '\0'; i ++)
+    uart_putc((unsigned char)str[i]);
+}
+
+void kernel_main(uint32_t r0, uint32_t r1, uint32_t atags)
+{
+  // Declare as unused
+  (void) r0;
+  (void) r1;
+  (void) atags;
+ 
+  uart_init();
+  uart_puts("Hello, kernel World!\r\n");
+ 
+  while (1)
+    uart_putc(uart_getc());
+}
-- 
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