--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/src/exec.c	Fri May 26 23:11:05 2017 +0900
@@ -0,0 +1,143 @@
+#include "types.h"
+#include "param.h"
+#include "defs.h"
+#include "memlayout.h"
+#include "mmu.h"
+#include "proc.h"
+#include "elf.h"
+#include "arm.h"
+
+// load a user program for execution
+int exec (char *path, char **argv)
+{
+    struct elfhdr elf;
+    struct inode *ip;
+    struct proghdr ph;
+    pde_t *pgdir;
+    pde_t *oldpgdir;
+    char *s;
+    char *last;
+    int i;
+    int off;
+    uint argc;
+    uint sz;
+    uint sp;
+    uint ustack[3 + MAXARG + 1];
+
+    if ((ip = namei(path)) == 0) {
+        return -1;
+    }
+
+    ilock(ip);
+
+    // Check ELF header
+    if (readi(ip, (char*) &elf, 0, sizeof(elf)) < sizeof(elf)) {
+        goto bad;
+    }
+
+    if (elf.magic != ELF_MAGIC) {
+        goto bad;
+    }
+
+    pgdir = 0;
+
+    if ((pgdir = kpt_alloc()) == 0) {
+        goto bad;
+    }
+
+    // Load program into memory.
+    sz = 0;
+
+    for (i = 0, off = elf.phoff; i < elf.phnum; i++, off += sizeof(ph)) {
+        if (readi(ip, (char*) &ph, off, sizeof(ph)) != sizeof(ph)) {
+            goto bad;
+        }
+
+        if (ph.type != ELF_PROG_LOAD) {
+            continue;
+        }
+
+        if (ph.memsz < ph.filesz) {
+            goto bad;
+        }
+
+        if ((sz = allocuvm(pgdir, sz, ph.vaddr + ph.memsz)) == 0) {
+            goto bad;
+        }
+
+        if (loaduvm(pgdir, (char*) ph.vaddr, ip, ph.off, ph.filesz) < 0) {
+            goto bad;
+        }
+    }
+
+    iunlockput(ip);
+    ip = 0;
+
+    // Allocate two pages at the next page boundary.
+    // Make the first inaccessible.  Use the second as the user stack.
+    sz = align_up (sz, PTE_SZ);
+
+    if ((sz = allocuvm(pgdir, sz, sz + 2 * PTE_SZ)) == 0) {
+        goto bad;
+    }
+
+    clearpteu(pgdir, (char*) (sz - 2 * PTE_SZ));
+
+    sp = sz;
+
+    // Push argument strings, prepare rest of stack in ustack.
+    for (argc = 0; argv[argc]; argc++) {
+        if (argc >= MAXARG) {
+            goto bad;
+        }
+
+        sp = (sp - (strlen(argv[argc]) + 1)) & ~3;
+
+        if (copyout(pgdir, sp, argv[argc], strlen(argv[argc]) + 1) < 0) {
+            goto bad;
+        }
+
+        ustack[argc] = sp;
+    }
+
+    ustack[argc] = 0;
+
+    // in ARM, parameters are passed in r0 and r1
+    proc->tf->r0 = argc;
+    proc->tf->r1 = sp - (argc + 1) * 4;
+
+    sp -= (argc + 1) * 4;
+
+    if (copyout(pgdir, sp, ustack, (argc + 1) * 4) < 0) {
+        goto bad;
+    }
+
+    // Save program name for debugging.
+    for (last = s = path; *s; s++) {
+        if (*s == '/') {
+            last = s + 1;
+        }
+    }
+
+    safestrcpy(proc->name, last, sizeof(proc->name));
+
+    // Commit to the user image.
+    oldpgdir = proc->pgdir;
+    proc->pgdir = pgdir;
+    proc->sz = sz;
+    proc->tf->pc = elf.entry;
+    proc->tf->sp_usr = sp;
+
+    switchuvm(proc);
+    freevm(oldpgdir);
+    return 0;
+
+    bad: if (pgdir) {
+        freevm(pgdir);
+    }
+
+    if (ip) {
+        iunlockput(ip);
+    }
+    return -1;
+}