Keyboard Work - setting up PLIC, UART, Interrupts

src/boot/entry.S

@@ -1,22 +1,20 @@
-.section .text
+.section .text.boot
 .global _start
 _start:
-    la sp, stack_top      # Set up the stack pointer
-    la t0, trap_entry     # Set up the trap handler
-    csrw mtvec, t0        # Set the trap vector to our trap handler
-    li t0, 0x00006000     # Load the mask for FS bits. enable float
-    csrs mstatus, t0      # Set the FS bits to 11 (Dirty/Initial). enable float
-    call kmain            # Jump to our C code
+    # 1. Set up the stack pointer using the symbol from our linker script
+    la sp, stack_top      
+
+    # 2. Set up the trap handler (pointing to the one in traps.S)
+    la t0, trap_entry     
+    csrw mtvec, t0        
+
+    # 3. Enable FPU (Floating Point)
+    li t0, 0x00006000     
+    csrs mstatus, t0      
+
+    # 4. Jump to C
+    call kmain            
+
 loop:
-    wfi                   # Wait for Interrupt (saves CPU)
-    j loop                # Infinite loop if C returns
-.align 4
-trap_entry:
-    csrr a0, mcause    # Argument 1: mcause
-    csrr a1, mepc      # Argument 2: mepc
-    j handle_trap
-.section .bss
-.align 16
-stack_low:
-    .skip 4096            # 4KB of stack space
-stack_top:
+    wfi                   
+    j loop

src/boot/traps.S

@@ -0,0 +1,83 @@
+# traps.S
+.section .text
+.align 4             # mtvec requires 4-byte alignment
+.global trap_entry
+
+trap_entry:
+    # 1. Create space on the stack for 32 registers (32 * 8 = 256 bytes)
+    addi sp, sp, -256
+
+    # 2. Save all General Purpose Registers (GPRs)
+    # We don't save x0 (zero) because it's always zero
+    sd ra, 0(sp)
+    sd gp, 8(sp)
+    sd tp, 16(sp)
+    sd t0, 24(sp)
+    sd t1, 32(sp)
+    sd t2, 40(sp)
+    sd s0, 48(sp)
+    sd s1, 56(sp)
+    sd a0, 64(sp)
+    sd a1, 72(sp)
+    sd a2, 80(sp)
+    sd a3, 88(sp)
+    sd a4, 96(sp)
+    sd a5, 104(sp)
+    sd a6, 112(sp)
+    sd a7, 120(sp)
+    sd s2, 128(sp)
+    sd s3, 136(sp)
+    sd s4, 144(sp)
+    sd s5, 152(sp)
+    sd s6, 160(sp)
+    sd s7, 168(sp)
+    sd s8, 176(sp)
+    sd s9, 184(sp)
+    sd s10, 192(sp)
+    sd s11, 200(sp)
+    sd t3, 208(sp)
+    sd t4, 216(sp)
+    sd t5, 224(sp)
+    sd t6, 232(sp)
+
+    # 3. Call your C handler
+    # The CPU already put the cause in 'mcause', so C can read it
+    call handle_trap
+
+    # 4. Restore all GPRs
+    ld ra, 0(sp)
+    ld gp, 8(sp)
+    ld tp, 16(sp)
+    ld t0, 24(sp)
+    ld t1, 32(sp)
+    ld t2, 40(sp)
+    ld s0, 48(sp)
+    ld s1, 56(sp)
+    ld a0, 64(sp)
+    ld a1, 72(sp)
+    ld a2, 80(sp)
+    ld a3, 88(sp)
+    ld a4, 96(sp)
+    ld a5, 104(sp)
+    ld a6, 112(sp)
+    ld a7, 120(sp)
+    ld s2, 128(sp)
+    ld s3, 136(sp)
+    ld s4, 144(sp)
+    ld s5, 152(sp)
+    ld s6, 160(sp)
+    ld s7, 168(sp)
+    ld s8, 176(sp)
+    ld s9, 184(sp)
+    ld s10, 192(sp)
+    ld s11, 200(sp)
+    ld t3, 208(sp)
+    ld t4, 216(sp)
+    ld t5, 224(sp)
+    ld t6, 232(sp)
+
+    # 5. Shrink the stack back
+    addi sp, sp, 256
+
+    # 6. Return from Machine-mode trap
+    mret

src/drivers/uart.c

@@ -1,14 +1,42 @@
 #include <stdint.h>
 #include <stddef.h>
 #include <stdarg.h>
+#include <syscon/syscon.h>
 #include <drivers/uart.h>
 #include <lib/string.h>
 
+void uart_init()
+{
+	kprint("UART Drive Init...");
+	volatile uint8_t *UART_IER = (uint8_t *)(UART_ADDRESS + 1);
+	*UART_IER = 0x01; // Enable "Received Data Available" interrupt
+	kputs("OK");
+}
+
 void uart_put(size_t base_addr, uint8_t data)
 {
 	*(volatile uint8_t *)base_addr = data;
 }
 
+char uart_getc()
+{
+	if (*UART_LSR & 0x01)
+	{
+		return (char)(*UART_RBR);
+	}
+	return '\0';
+}
+
+void uart_handle_interrupt()
+{
+	char c = uart_getc();
+	if (c != '\0')
+	{
+		// Later, this will go into a "Circular Buffer"
+		kputchar(c);
+	}
+}
+
 int kputchar(int ch)
 {
 	uart_put(UART_ADDRESS, ch);
@@ -107,6 +135,11 @@ void kprintf(const char *format, ...)
 			p++;
 			switch (*p)
 			{
+			case 'c':
+			{
+				char c = (char)va_arg(args, int);
+				kputchar(c);
+			}
 			case 's':
 			{
 				char *s = va_arg(args, char *);

src/drivers/uart.h

@@ -1,8 +1,19 @@
 #ifndef UART_H
 #define UART_H
 
+#define UART_RBR (volatile uint8_t *)(UART_ADDRESS + 0)
+#define UART_LSR (volatile uint8_t *)(UART_ADDRESS + 5)
 #define UART_ADDRESS 0x10000000
+#define UART_INTERRUPT_ENABLE_REGISTER 0x1001100C
+#define UART_INTEN_OFFSET 0x0C
+#define UART_TX_OFFSET 0x04
+#define UART_INTERRUPT_ENABLE_REGISTER (UART_BASE_ADDR + UART_INTEN_OFFSET)
+
+void uart_init();
 void uart_put(size_t base_addr, uint8_t data);
+char uart_getc();
+void uart_handle_interrupt();
+
 int kputchar(int ch);
 void kprint_hex(uint64_t val);
 void kprint_int(int num);

src/kernel/interrupts.c

@@ -2,7 +2,24 @@
 #include <stdint.h>
 #include <drivers/uart.h>
 #include <syscon/syscon.h>
-#include "memory.h"
+#include <kernel/plic.h>
+#include <kernel/interrupts.h>
+#include <kernel/memory.h>
+
+void interrupt_init()
+{
+    kprint("Initialising Interrupts...");
+    uint64_t mstatus_val;
+    asm volatile("csrr %0, mstatus" : "=r"(mstatus_val));
+    mstatus_val |= (1 << MSTATUS_BIT_MIE);
+    asm volatile("csrw mstatus, %0" ::"r"(mstatus_val));
+
+    uint64_t mie_val;
+    asm volatile("csrr %0, mie" : "=r"(mie_val));
+    mie_val |= (1 << MIE_BIT_MEIE); //|(1 << MIE_BIT_MTIE);
+    asm volatile("csrw mie, %0" ::"r"(mie_val));
+    kputs("OK");
+}
 
 void kpanic(const char *reason)
 {
@@ -31,12 +48,22 @@ void kpanic_force()
 
 void handle_trap()
 {
-    kprint("\n!!! HARDWARE EXCEPTION DETECTED !!!\n");
-
     // Read the 'mcause' register to see WHY we trapped
     unsigned long cause;
     __asm__ volatile("csrr %0, mcause" : "=r"(cause));
 
+    // Check if the top bit is 1 (Interrupt) or 0 (Exception)
+    // For 64-bit RISC-V, the bit is 63
+    int is_interrupt = (cause >> 63) & 1;
+
+    if (is_interrupt)
+    {
+        unsigned long code = cause & 0xfff;
+        handle_interrupt(code);
+        return;
+    }
+    else
+    {
         // fault address (if applicable)
         uintptr_t mtval;
         asm volatile("csrr %0, mtval" : "=r"(mtval));
@@ -44,35 +71,51 @@ void handle_trap()
         switch (cause)
         {
         case 0:
-        kprint("Reason: Instruction Address Misaligned\n");
+            kpanic("Reason: Instruction Address Misaligned\n");
             break;
         case 1:
-        kprint("Reason: Instruction Access Fault\n");
+            kpanic("Reason: Instruction Access Fault\n");
             break;
         case 2:
-        kprint("Reason: Illegal Instruction\n");
+            kpanic("Reason: Illegal Instruction\n");
             break;
         case 3:
-        kprint("Reason: Breakpoint (ebreak)\n");
+            kpanic("Reason: Breakpoint (ebreak)\n");
             break;
         case 4:
-        kprint("Reason: Load Address Misaligned\n");
+            kpanic("Reason: Load Address Misaligned\n");
             break;
         case 5:
-        kprint("Reason: Load Access Fault\n");
+            kpanic("Reason: Load Access Fault\n");
             break;
         case 6:
-        kprint("Reason: Store/AMO Address Misaligned\n");
+            kpanic("Reason: Store/AMO Address Misaligned\n");
             break;
         case 7:
-        kprint("Reason: Store/AMO Access Fault\n");
+            kpanic("Reason: Store/AMO Access Fault\n");
             break;
         default:
-        kprintf("Reason: Unknown Exception Code %d\n", cause);
+            break;
+            // kpanic("Reason: Unknown Exception Code %d\n", cause);
         }
         kprintf("Faulting Address (if applicable): %x\n", mtval);
-
-    heap_stats();
-
-    poweroff();
+    }
+}
+
+void handle_interrupt(unsigned long code)
+{
+    switch (code)
+    {
+    case 7:
+        break; // timer Interrupt. Ignoring for now.
+    case 11:
+        volatile uint32_t *claim_reg = (uint32_t *)PLIC_CLAIM_COMPLETE;
+        uint32_t irq = *claim_reg;
+        if (irq == 10)
+        {
+            uart_handle_interrupt();
+        }
+        *claim_reg = irq;
+        break;
+    }
 }

src/kernel/interrupts.h

@@ -1,8 +1,10 @@
 #ifndef PANIC_H
 #define PANIC_H
 
-void kpanic(const char *reason);
+void interrupt_init();
+void kpanic(const char *);
 void kpanic_force();
+void handle_interrupt(unsigned long code);
 
 #define KASSERT(cond, msg)                                              \
     if (!(cond))                                                        \

src/kernel/kernel.c

@@ -2,6 +2,7 @@
 #include <stddef.h>
 #include <drivers/uart.h>
 #include <syscon/syscon.h>
+#include <kernel/plic.h>
 #include <kernel/interrupts.h>
 #include <kernel/memory.h>
 
@@ -11,9 +12,8 @@ void kmain()
     kprintf("Hello, from %s!", "SquidgeOS");
     kputs("----------------------");
     knewline();
+    plic_init();
+    uart_init();
     page_init();
-    test_memory_integrity();
-    test_memory_alignment();
-    test_memory_stress();
-    poweroff();
+    interrupt_init();
 }

src/kernel/memory.c

@@ -11,7 +11,7 @@ extern uint8_t _heap_start[]; // named in the linker script
 
 void page_init()
 {
-	kprint("Initialising page allocator.\n");
+	kprint("Initialising page allocator...");
 	uintptr_t start = ((uintptr_t)_heap_start + PAGE_SIZE - 1) & ~(PAGE_SIZE - 1);
 	uintptr_t end = 0x88000000; // Default QEMU RAM limit
 
@@ -19,6 +19,7 @@ void page_init()
 	{
 		page_free((void *)addr);
 	}
+	kputs("OK");
 }
 
 void page_free(void *addr)

src/kernel/plic.c

@@ -0,0 +1,19 @@
+#include <stdint.h>
+#include "kernel/plic.h"
+
+void plic_init()
+{
+    int hart = 0;
+
+    // 1. Set the priority of the UART interrupt
+    // We set it to 1. If it's 0 (the default), the interrupt is effectively disabled.
+    *PLIC_PRIORITY(UART_IRQ) = 1;
+
+    // 2. Enable the UART interrupt for Hart 0
+    // This is a bitmask, so we shift 1 by the IRQ number.
+    *PLIC_ENABLE(hart) = (1 << UART_IRQ);
+
+    // 3. Set the priority threshold for Hart 0
+    // We set this to 0 so that ANY interrupt with priority > 0 gets through.
+    *PLIC_THRESHOLD(hart) = 0;
+}

src/kernel/plic.h

@@ -0,0 +1,23 @@
+#ifndef PLIC_H
+#define PLIC_H
+
+#define PLIC_BASE 0x0c000000
+#define PLIC_CLAIM_COMPLETE 0x0c200004
+
+// Priorities: 4 bytes per IRQ (IRQ 0 is reserved/null)
+#define PLIC_PRIORITY(irq) ((volatile uint32_t *)(PLIC_BASE + (irq) * 4))
+
+// Enables: Each Hart has a 0x80 byte stride for its enable bits
+// For Hart 0 Machine Mode: 0x0c002000
+#define PLIC_ENABLE(hart) ((volatile uint32_t *)(PLIC_BASE + 0x2000 + (hart) * 0x80))
+
+// Threshold and Claim/Complete: Each Hart has a 0x1000 byte stride
+// For Hart 0 Machine Mode: 0x0c200000 and 0x0c200004
+#define PLIC_THRESHOLD(hart) ((volatile uint32_t *)(PLIC_BASE + 0x200000 + (hart) * 0x1000))
+#define PLIC_CLAIM(hart) ((volatile uint32_t *)(PLIC_BASE + 0x200004 + (hart) * 0x1000))
+
+#define UART_IRQ 10
+
+void plic_init();
+
+#endif

src/lib/string.c

@@ -23,7 +23,8 @@ void *memcpy(void* dest, const void* src, size_t size)
 }
 
 int strcmp(const char *str1, const char *str2)
-{    while(*str1 == *str2)
+{
+    while (*str1 == *str2)
     {
         if (*str1 == '\0')
         {
@@ -38,7 +39,8 @@ int strcmp(const char * str1, const char * str2)
 size_t strlen(const char *str)
 {
     size_t c = 0;
-    while(*str++ != '\0'){
+    while (*str++ != '\0')
+    {
         c++;
     }
     return c;

src/syscon/syscon.h

@@ -8,6 +8,18 @@
 #define SYSCON_POWEROFF 0x5555
 #define SYSCON_REBOOT 0x7777
 
+#define write_register(register, value) \
+    asm volatile("csrrw zero, %0, %1" ::"i"(register), "r"(value))
+
+#define read_register(register, destination) \
+    asm volatile("csrrs %0, %1, zero" : "=r"(destination) : "i"(register))
+
+#define MSTATUS 0x300
+#define MIE 0x304
+#define MSTATUS_BIT_MIE 3
+#define MIE_BIT_MTIE 7
+#define MIE_BIT_MEIE 11
+
 void poweroff(void);
 void reboot(void);