Add missing files

os/process.c

@@ -0,0 +1,15 @@
+#include "process.h"
+#include "alloc.h"
+
+void create_process_table(ProcessTable **table) {
+    *table = malloc(sizeof(ProcessTable));
+    (*table)->entries = 0;
+}
+
+int create_process(ProcessTable *table, void *entrypoint) {
+    if (table->entries>=MAX_PROCESS) return 1;
+
+    Process *pentry = &table->process_list[table->entries++];
+    pentry->entrypoint = entrypoint;
+    return 0;
+}

os/process.h

@@ -0,0 +1,23 @@
+#ifndef PROCESS_H
+#include <stdint.h>
+
+#define PROCESS_H
+#define MAX_PROCESS 16
+
+#define STACK_SIZE 32
+#define STACK_START(stack)  (&stack[STACK_SIZE-1])
+
+typedef struct {
+    int (*entrypoint)();
+    uint32_t stack[STACK_SIZE];
+} Process;
+
+typedef struct {
+    Process process_list[MAX_PROCESS];
+    int entries;
+} ProcessTable;
+
+void create_process_table(ProcessTable **table);
+
+int create_process(ProcessTable *table, void *entrypoint);
+#endif

os/scheduler.c

@@ -0,0 +1,31 @@
+#include "process.h"
+
+uint32_t **current_sp;
+
+extern void puts(const char *s);
+
+void start_task(Process *pentry) {
+    __asm volatile(
+        "mov %0, sp"
+        : "=r" (*current_sp)
+    );
+    __asm volatile(
+        "mov sp, %0\n"
+        :: "r" (STACK_START(pentry->stack))
+    );
+    pentry->entrypoint();
+    __asm volatile(
+        "mov sp, %0\n"
+        :: "r" (*current_sp)
+    );
+}
+
+void run(ProcessTable *ptable) {
+    while(1) {
+        for (int i = 0; i<ptable->entries; i++) {
+            Process *pentry = &ptable->process_list[i];
+
+            start_task(pentry);
+        }
+    }
+}

os/scheduler.h

@@ -0,0 +1 @@
+void run();

startup.s

@@ -0,0 +1,15 @@
+// The .word directive allocates a 32-bit value in the memory. In this case it allocates in memory
+// the address of the stack_top (value in the linker.ld file) and _Reset.
+.word stack_top     // Address of the stack_top
+.word _start        // Address of the _start function
+
+// The thumb_func is used to make sure the function is in thumb mode,
+// which is required for the Cortex-M0+.
+.thumb_func
+.syntax unified
+
+.global _start
+
+_start:
+    BL main
+    B .

tasks/tasks.c

@@ -0,0 +1,11 @@
+#include "library.h"
+
+int task2(void) {
+    puts("Hello World from task 1\n");
+    return 0;
+}
+
+int task1(void) {
+    puts("Hello World from task 2\n");
+    return 0;
+}