Adding a list of integers

1
Adding a list of integers

• Here apply our previous lessons about
  command-line arguments and numeric conversions

2
Our add.s demo

• This program allows a user to type a list of
  unsigned integers as program arguments
• Example ./add 11 22 33
• Then the program converts these strings of
  digit-characters into the integer values they
  represent, adds those integers, does a
  number-to-string conversion and shows us the
  result The total is 66

3
The stack upon entry
command-line
./add 11 22 33
applications stack
NULL
argv3
argv2
argv1
argv0
4
SSRSP
4
Symbols and data
manifest constants (to make code
understandable) .equ STDOUT, 1 device-file
ID-number .equ sys_write, 1 system-call
ID-number .equ sys_exit, 60 system-call
ID-number static data (for storing data
values in memory) .section .data ten .quad 10
the decimal-system radix total .quad 0 for
sum of the arguments msg .ascii The total is
title for programs output buf .ascii
\n buffer for the result-string len .
quad . msg length of output message
5
atoi and itoa

• We employ a pair of subroutines that will perform
  the needed data-conversions
• From a digit-string to an unsigned integer
• From an unsigned integer to a digit-string
• We call atoi for doing ascii-to-integer
• We call itoa for doing integer-to-ascii

6
Project 1

• Can you design modifications of these two
  subroutines so that the add.s program is able
  to correctly handle signed integers?
• Example
• ./add 115 -50 10 -200
• The total is -125

7
unsigned versus signed
8
7
9
-8
7
-7
6
10
6
-6
5
11
5
-5
12
4
-4
4
13
3
-3
3
2
14
2
-2
1
15
1
-1
0
0
8
Unsigned interpretation
7
6
5
4
3
2
1
0
Position
128
64
32
16
8
4
2
1
Weight
1
0
0
1
1
1
0
1
Digit
128 0 0 16 8 4 0 1 157
9
Signed interpretation
7
6
5
4
3
2
1
0
Position
-128
64
32
16
8
4
2
1
Weight
1
0
0
1
1
1
0
1
Digit
-128 0 0 16 8 4 0 1 - 99
10
Negating an 8-bit integer

• For the twos complement representation of
  signed integers, you can quickly obtain the
  negative of a number by applying this two-step
  rule Flip the bits, then add one

Example negative seven 11111001 Flip the
bits 00000110 Now add one
1 -------------------------- Result
positive seven 00000111