+ Argument registers (caller-saves)
+ - %r26, %r25, %r24, %r23, %r22
+ * In asm input.
+
+ Return registers (caller-saves)
+ - %r28 (return), %r21 (errno)
+ * In asm output.
+
+ Caller-saves registers
+ - %r1, %r27, %r29
+ - %r2 (return pointer)
+ - %r31 (ble link register)
+ * In asm clobber.
+
+ Callee-saves registers
+ - %r3-%r18
+ - %r30 (stack pointer)
+ * Not in asm clobber.
+
+ If userspace is 32-bit:
+ Callee-saves registers
+ - %r19 (32-bit PIC register)
+
+ Differences from 32-bit calling convention:
+ - Syscall number in %r20
+ - Additional argument register %r22 (arg4)
+ - Callee-saves %r19.
+
+ If userspace is 64-bit:
+ Callee-saves registers
+ - %r27 (64-bit PIC register)
+
+ Differences from 64-bit calling convention:
+ - Syscall number in %r20
+ - Additional argument register %r22 (arg4)
+ - Callee-saves %r27.