kernel.c


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#include <uart.h>

void kernel_main(uint32_t r0, uint32_t r1, uint32_t atags)
{
  // Declare as unused
  (void) r0;
  (void) r1;
  (void) atags;

  uart_init();

  // When we attach screen session after loading kernel with socat
  // we miss kernel's greeting... So we'll make the kernel wait for
  // one char we're going to send from within screen
  uart_getc();
  
  uart_puts("Hello, kernel World!\r\n");

  uint32_t ID_MMFR0;
  // get contents of coprocessor register to check for paging support
  asm("mrc p15, 0, %0, c0, c1, 4" : "=r" (ID_MMFR0));

  char *paging;

  switch(ID_MMFR0 & 0xf) /* lowest 4 bits indicate VMSA support */ {
  case 0 : paging = "no paging\n\r"; break;
  case 1 : paging = "implementation defined paging\n\r"; break;
  case 2 : paging = "VMSAv6, with cache and TLB type registers\n\r"; break;
  case 3 : paging = "VMSAv7, with support for remapping and access flag\n\r"; break;
  case 4 : paging = "VMSAv7 with PXN bit supported\n\r"; break;
  case 5 : paging = "VMSAv7, PXN and long format descriptors. EPAE is supported.\n\r"; break;
  default : paging = "?_? unknown paging ?_?\n\r";
  }

  uart_puts(paging);

  uint32_t CPSR;
  // get content of current program status register to check the current
  // processor mode
  asm("mrs %0, cpsr" : "=r" (CPSR) :: "memory");

  char *mode;
  
  switch(CPSR & 0x1f) /* lowest 5 bits indicate processor mode */ {
  case 0x10 : mode = "User (PL0)"; break;
  case 0x11 : mode = "FIQ (PL1)"; break;
  case 0x12 : mode = "IRQ (PL1)"; break;
  case 0x13 : mode = "Supervisor (PL1)"; break;
  case 0x16 : mode = "Monitor (PL1)"; break;
  case 0x17 : mode = "Abort (PL1)"; break;
  case 0x1a : mode = "Hyp (PL2)"; break;
  case 0x1b : mode = "Undefined (PL1)"; break;
  case 0x1f : mode = "System (PL1)"; break;
  default : mode = "Unknown mode"; break;
  }

  uart_puts(mode);
  
  while (1)
    uart_putc(uart_getc());
}