Minimal printk in C

Makefile

@@ -1,6 +1,6 @@
 .POSIX:
 CCX=arm-none-eabi-g++
-CCFLAGS=-c -Wall -fno-stack-protector -ffreestanding -march=armv7-a -nostdlib -Iinclude -g
+CCFLAGS=-c -Wall -fno-stack-protector -ffreestanding -fno-exceptions -march=armv7-a -nostdlib -Iinclude -g
 
 C_HDR=$(wildcard include/*.h)
 C_SRC=$(wildcard src/*.cpp)
@@ -19,7 +19,6 @@ environ:
 
 bin/kernel.elf: environ $(A_OBJ) $(C_OBJ)
 	arm-none-eabi-ld --nmagic -nostdlib -T src/linker.ld -o bin/kernel.elf obj/*.o
-	# maybe -msomearch
 
 obj/s_%.o: src/%.s
 	$(CCX) $(CCFLAGS) -o $@ $<

src/kernel.cpp

@@ -1,3 +1,33 @@
-extern "C" int f(int a) {
+#define UART_RBR    0x01c28000
-	return a + 1;
+char* const UART_THR = (char* const)0x01c28000;
+#define UART_IER    0x01c28004
+char* const UART_LSR = (char* const)0x01c28014;
+
+extern "C" int printk(const char*);
+
+extern "C" int main(int argc, char** argv) {
+    char letter[2] = "A";
+
+    for (char c = 'A'; c < 'Z'; c++) {
+        letter[0] = c;
+
+        printk("HELLO WORLD!");
+        printk("\t");
+        printk(letter);
+        printk("\r\n");
+    }
+
+   return 0;
 }
+
+/* Never call this function directly: always use the printk,
+ * which disables and reenables interrupts */
+extern "C" int c_printk(const char* msg) {
+    while (*msg != '\0') {
+        while ((*UART_LSR & 0x20) == 0);    /* Wait for transmission */
+        *UART_THR = *msg;
+        msg++;
+    }
+    return 0;
+}
+

src/kernel.s

@@ -1,4 +1,3 @@
-.extern f
 
 .text
 my_string:
@@ -9,57 +8,28 @@ my_string:
 .align 4
 .global _start
 _start:
-    /* does nothing special */
+    bl main
-	mov r13, #0x8000
-
-    mov r1, #16
-
-1:
-    ldr r0, =my_string
-    bl printk
-    sub r1, #1
-    cmp r1, #0
-    b 1b
-
-lup:
-    b lup
 
 
+
+
+endless_busy_loop:
+    cpsid if
+    b endless_busy_loop
+
+.extern c_printk
 .global printk
 printk:
-    stmfd r13!, {r0, r1, r2, r3, r4, r14}
+    push {lr}
 
-    /* disable UART interrupts */
+    /* disables IRQ and FIR in CPU */
-    mov r2, #0x8004
-    movt r2, #0x01c2
-
-    ldr r1, [r2]
-    and r1, #0xfffffffc
-    str r1, [r2]
-
-    /* mask interrupts in CPU */
     cpsid if
 
-    /* TBR */
-    mov r2, #0x8000
-    movt r2, #0x01c2
 
-1:
+    bl c_printk
-    ldr r1, [r0]
-    add r0, #4
 
-    mov r4, #4
+    /* enables IRQ and FIR in CPU */
-2:
+    cpsie if
-    and r3, r1, #0xff
-    mov r1, r1, lsr#8
-    cmp r3, #0
-    beq 3f
-    str r3, [r2]
-    sub r4, #1
-    cmp r4, #0
-    bne 2b
-    b 1b
-3:
 
-    ldmfd r13!, {r0, r1, r2, r3, r4, r15}
+    pop {pc}