Support preserved registers as identifiers

… in expressions

boot/common-boot.s

@@ -1,5 +1,6 @@
 .global _is_alpha, _is_digit, _is_alnum, _is_upper, _is_lower
-.global _write_out, _read_file, _memcmp, _write_error, _put_char, _printi
+.global _write_out, _read_file, _write_error, _put_char, _printi
+.global _get, _memcmp
 .global _divide_by_zero_error, _exit
 
 .section .rodata
@@ -187,7 +188,9 @@ _write_out:
 # a0 - Buffer pointer.
 # a1 - Buffer size.
 #
-# Returns the result in a0.
+# Sets s1 to the buffer passed in a0.
+#
+# Returns the amount of bytes written in a0.
 .type _read_file, @function
 _read_file:
 	# Prologue.
@@ -196,9 +199,11 @@ _read_file:
 	sw s0, 0(sp)
 	addi s0, sp, 8
 
-	mv a2, a1
-	mv a1, a0
+	mv s1, a0
+
 	li a0, STDIN
+	mv a2, a1
+	mv a1, s1
 	li a7, SYS_READ
 	ecall
 
@@ -293,3 +298,11 @@ _put_char:
 	lw s0, 8(sp)
 	add sp, sp, 16
 	ret
+
+# a0 - Pointer to an array to get the first element.
+#
+# Dereferences a pointer and returns what is on the address in a0.
+.type _get, @function
+_get:
+	lw a0, (a0)
+	ret