Adding a list of integers
Here apply our previous lessons
about command-line arguments
and numeric conversions
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’
The stack upon entry
command-line:
$ ./add 11 22 33
application’s stack
NULL
argv[3]
argv[2]
argv[1]
argv[0]
SS:RSP
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
ten:
total:
msg:
buf:
len:
# static data (for storing data values in memory)
.section .data
.quad
10
# the decimal-system radix
.quad
0
# for sum of the arguments
.ascii
“The total is “
# title for program’s output
.ascii
“
\n”
# buffer for the result-string
.quad
. – msg
# length of output message
‘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
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
‘unsigned’ versus ‘signed’
9
8
7
-7
6
10
11
12
3
2
15
0
1
6
-5
4
14
7
-6
5
13
-8
5
-4
4
-3
3
2
-2
-1
0
1
Unsigned interpretation
Position:
7
6
5
4
3
2
1
0
Weight:
128
64
32
16
8
4
2
1
1
0
0
1
1
1
0
1
Digit:
128 + 0 + 0 + 16 + 8 + 4 + 0 + 1 = 157
Signed interpretation
Position:
7
6
5
4
3
2
1
0
Weight:
-128
64
32
16
8
4
2
1
1
0
0
1
1
1
0
1
Digit:
-128 + 0 + 0 + 16 + 8 + 4 + 0 + 1 = - 99
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:
Flip the bits:
Now add one:
Result:
negative seven = 11111001
00000110
+1
-------------------------positive seven = 00000111