Implement simple binary expressions

boot/common-boot.s

@@ -1,4 +1,5 @@
-.global _is_alpha, _is_digit, _is_alnum, _is_upper, _is_lower, _write_out, _read_file, exit, _memcmp, _write_error
+.global _is_alpha, _is_digit, _is_alnum, _is_upper, _is_lower
+.global _write_out, _read_file, exit, _memcmp, _write_error, _put_char
 
 .section .rodata
 
@@ -200,6 +201,29 @@ _read_file:
 	ret
 
 # Terminates the program. a0 contains the return code.
+.type _exit, @function
 exit:
 	li a7, SYS_EXIT
 	ecall
+
+# Writes a character from a0 into the standard output.
+.type _put_char, @function
+_put_char:
+	# Prologue
+	addi sp, sp, -16
+	sw ra, 12(sp)
+	sw s0, 8(sp)
+	addi s0, sp, 16
+
+	sb a0, 4(sp)
+	li a0, STDOUT
+	addi a1, sp, 4
+	li a2, 1
+	li a7, SYS_WRITE
+	ecall
+
+	# Epilogue.
+	lw ra, 12(sp)
+	lw s0, 8(sp)
+	add sp, sp, 16
+	ret