Implement if statements

boot/common-boot.s

@@ -1,5 +1,6 @@
 .global _is_alpha, _is_digit, _is_alnum, _is_upper, _is_lower
-.global _write_out, _read_file, exit, _memcmp, _write_error, _put_char
+.global _write_out, _read_file, _memcmp, _write_error, _put_char, _printi
+.global _divide_by_zero_error, _exit
 
 .section .rodata
 
@@ -119,6 +120,13 @@ _is_alpha:
 	addi sp, sp, 16
 	ret
 
+# Detects whether the passed character is a digit
+# (a value between 0 and 9).
+#
+# Parameters:
+# a0 - Exemined value.
+#
+# Sets a0 to 1 if it is a digit, to 0 otherwise.
 .type _is_digit, @function
 _is_digit:
 	li t0, '0' - 1
@@ -201,10 +209,68 @@ _read_file:
 	ret
 
 # Terminates the program. a0 contains the return code.
+#
+# Parameters:
+# a0 - Status code.
 .type _exit, @function
-exit:
+_exit:
 	li a7, SYS_EXIT
 	ecall
+	# ret
+
+.type _divide_by_zero_error, @function
+_divide_by_zero_error:
+    addi a7, zero, 172 # getpid
+    ecall
+
+    addi a1, zero, 8 # SIGFPE
+    addi a7, zero, 129 # kill
+    ecall
+    ret
+
+# a0 - Whole number.
+# t1 - Constant 10.
+# a1 - Local buffer.
+# t2 - Current character.
+# t3 - Whether the number is negative.
+.type printi, @function
+_printi:
+    addi sp, sp, -16
+    sw s0, 0(sp)
+    sw ra, 4(sp)
+    addi s0, sp, 16
+    addi t1, zero, 10
+    addi a1, s0, -1
+
+    addi t3, zero, 0
+    bge a0, zero, .digit10
+    addi t3, zero, 1
+    sub a0, zero, a0
+
+.digit10:
+    rem t2, a0, t1
+    addi t2, t2, '0'
+    sb t2, 0(a1)
+    div a0, a0, t1
+    addi a1, a1, -1
+    bne zero, a0, .digit10
+
+    beq zero, t3, .write_call
+    addi t2, zero, '-'
+    sb t2, 0(a1)
+    addi a1, a1, -1
+
+.write_call:
+    addi a0, zero, 1
+    addi a1, a1, 1
+    sub a2, s0, a1
+    addi a7, zero, 64 # write
+    ecall
+
+    lw s0, 0(sp)
+    lw ra, 4(sp)
+    addi sp, sp, 16
+    ret
 
 # Writes a character from a0 into the standard output.
 .type _put_char, @function