Herbert & Brian,
You should NOT think about indexed addressing in terms of records.
Records are typically buffered one at a time and not addressed by indexing.
It is better to see indexed addressing in terms of accessing arrays.
Say you have an array of 20 integers in memory.
(One integer = 4 bytes)
Now you can acces these in a loop, by using an index and the fixed
multiplier of 4 (4 bytes is the size of one element of the array).
so 1st element: index = 0 in %edi, address with "start_array (,%edi,4)"
next element: increment %edi and use same statement ("start_array(,%edi,4)")
etc.
When you have an array of characters (1 byte) then your multiplier is 1.
Hope this clarifies, if not, do not worry about it, it will become
clearer later in the book, just carry on!
success
GijsH
On Sun, Mar 7, 2010 at 12:46 PM, Herbert R Coburn <address@hidden
<mailto:address@hidden>> wrote:
On 03/06/2010 11:04 PM, Brian Haag wrote:
Thanks very much for the response! I think I follow you, but it
doesn't
appear to be what the book says... but I think maybe the text of the
book is just unclear on this point.
Here is a sample record that starts at 2002 (I'm in the USA)
Name: 30 bytes -- starts at 0
Addr: 30 bytes -- starts at 30
City: 25 bytes -- starts at 60
State: 2 bytes -- starts at 85
Zip: 9 bytes -- starts at 87
Phone: 10 bytes -- starts at 96
Record Length is 106.
To find the City for the 7th record:
I want to skip 6 records so the multiplier is 6.
Each record is 106 bytes long, so the index is 106.
The City field starts at the 60th byte, so add 60 to the
address.
Ergo, use (2002+60) for the address, 106 as the index, and
(7-1) as
the multiplier. That's 2062, 106, 6.
<http://lists.nongnu.org/mailman/listinfo/pgubook-readers>
p 15 of the book is stating that I have three quantities to work
with:
an address, an index register, and a multiplier:
"For example, we could specify address 2002 and an index
register. If
the index register contains the number 4, the actual address the
data is
loaded from would be 2006. This way, if you have a set of numbers
starting at location 2002, you can cycle between each of them
using an
index register."
What does "cycle" mean here? That I can increment the index
register
and get a different byte every time I access memory using this
method?
Is this a different use than the one you described? If the
length of
a record is 1 byte, then I think you'd prescribe leaving the index
register at 1 and incrementing the multiplier, right?
"On x86 processors, you can also specify a multiplier for the index.
This allows you to access memory a byte at a time or a word at a
time (4
bytes). If you are accessing an entire word, your index will need to
be multiplied by 4 to get the exact location of the fourth
element from
your address. For example, if you wanted to access the fourth
byte from
location 2002, you would load your index register with 3
(remember, we
start counting at 0) and set the multiplier to 1 since you are
going a
byte at a time. This would get you
location 2005. However, if you wanted to access the fourth word from
location 2002, you would load your index register with 3 and set the
multiplier to 4. This would load from location 2014 - the fourth
word."
This is what confuses me. I start with an address of 2002, an
index of
3, and a multiplier of 1. This gets me location 2005. The text
(to me)
reads as "set the multiplier to the length of the record" ("a
byte at a
time").
See my confusion? Thanks.
Cycle means loop through, incrementing the index register according to
the what you quoted; incrementing the multiplier according to what I
wrote.
Now, I'm confused. Am I possibly wrong? Someone else will need to
clarify.
/herbc