Add SYS_putc and SYS_puts.

Makefile

@@ -69,8 +69,8 @@ debug-nox: image.bin
 		-ex "break _start" \
 		-ex "continue"
 
-fs.img: kernel.bin tools/mkfs user/false user/greet user/div0
+fs.img: kernel.bin tools/mkfs user/false user/greet user/div0 user/shout
-	tools/mkfs $@ $< user/false user/greet user/div0
+	tools/mkfs $@ $< user/false user/greet user/div0 user/shout
 
 LDFLAGS=-m elf_i386
 

cpu/idt.c

@@ -109,6 +109,21 @@ void trap(registers_t *r) {
     }
 }
 
+static void* get_userspace_ptr(uint32_t ptr) {
+    if (ptr >= 0xffffffff - USER_BASE) {
+        return 0;
+    }
+    return (void*)(ptr + USER_BASE);
+}
+
+static int handle_puts(const char* s) {
+    if (!s) {
+        return -1;
+    }
+    printk(s);
+    return 0;
+}
+
 static void handle_syscall(registers_t* r) {
     switch (r->eax) {
         case SYS_exit:
@@ -122,6 +137,13 @@ static void handle_syscall(registers_t* r) {
             printk("Hello world!\n");
             r->eax = 0;
             break;
+        case SYS_putc:
+            printk((const char[]){r->ebx, '\0'});
+            r->eax = 0;
+            break;
+        case SYS_puts:
+            r->eax = handle_puts(get_userspace_ptr(r->ebx));
+            break;
         default:
             printk("Unknown syscall\n");
             r->eax = -1;

syscall.h

@@ -4,6 +4,8 @@ enum {
 	T_SYSCALL = 0x84,
 	SYS_exit = 0,
 	SYS_greet = 1,
+	SYS_putc = 2,
+	SYS_puts = 3,
 };
 
 int syscall(int call, int arg);

user/shout.c

@@ -0,0 +1,16 @@
+#include "../syscall.h"
+#include <stdint.h>
+
+void userspace_puts(const char* s) {
+    char c;
+    while ((c = *s++)) {
+        syscall(SYS_putc, c);
+    }
+}
+
+int main() {
+    const char* spell = "cra cra trif traf not sgnieflet\n";
+    userspace_puts(spell);
+    syscall(SYS_puts, (uint32_t)spell);
+    return 0;
+}