arm: thumb understanding++

userland/arch/arm/dump_regs.c

@@ -0,0 +1,16 @@
+/* Dump ARM registers that can be read in EL0 (and higher). */
+
+#include <stdio.h>
+#include <inttypes.h>
+
+int main(void) {
+    uint32_t cpsr;
+    uint32_t cpsr_m;
+    __asm__ ("mrs %0, cpsr" : "=r" (cpsr) : :);
+    printf("CPSR 0x%" PRIX32 "\n", cpsr);
+    /* https://github.com/cirosantilli/linux-kernel-module-cheat#arm-exception-levels */
+    cpsr_m = cpsr & 0xF;
+    printf("CPSR.M 0x%" PRIX32 "\n", cpsr_m);
+
+    return 0;
+}

userland/arch/arm/freestanding/linux/hello.S

@@ -6,7 +6,6 @@
 .text
 .global _start
 _start:
-asm_main_after_prologue:
     /* write */
     mov r0, 1     /* stdout */
     adr r1, msg   /* buffer */

userland/arch/arm/freestanding/linux/hello_thumb.S

@@ -0,0 +1,22 @@
+/* https://github.com/cirosantilli/linux-kernel-module-cheat#arm-thumb-encoding */
+
+.thumb_func
+.syntax unified
+.text
+.global _start
+_start:
+asm_main_after_prologue:
+    /* write */
+    mov r0, 1     /* stdout */
+    adr r1, msg   /* buffer */
+    ldr r2, =len  /* len */
+    mov r7, 4     /* syscall number */
+    svc 0
+
+    /* exit */
+    mov r0, 0     /* exit status */
+    mov r7, 1     /* syscall number */
+    svc 0
+msg:
+    .ascii "hello\n"
+len = . - msg

userland/arch/arm/thumb.S

@@ -1,21 +1,12 @@
-/* https://github.com/cirosantilli/linux-kernel-module-cheat#arm-instruction-encodings
- *
- * Illustrates features that are only available in thumb.
- * TODO ensure that we are actually inside of thumb.
- */
+/* https://github.com/cirosantilli/linux-kernel-module-cheat#arm-thumb-encoding */
+
+#include <lkmc.h>
 
-.syntax unified
-.text
 .thumb_func
-.global main
-main:
-main_after_prologue:
+LKMC_PROLOGUE
 
-    /* CBZ: cmp and branch if zero instruction. Equivalent to CMP + BEQ.
-     * TODO create an interesting assertion here.
-     */
+    /* TODO: #if 0 something that is not thumb encodable. */
     cbz r1, 1f
     1:
 
-    mov r0, 0
-    bx lr
+LKMC_EPILOGUE