That is, it uses the current value of the AL register as an index into the array whose base address is found in EBX. It fetches the byte at that index in the array and copies that byte into the AL register. Intel calls this instruction translate because programmers typically use it to translate characters from one form to another using a lookup table. That's exactly how we are using it here. In the previous example, CnvrtLower is a 256-byte table that contains the values 0..$60 at indices 0..$60, $41.$5A at indices $61.$7A, and $7B..$FF at indices $7Bh.OFF. Therefore, if AL contains a value in the range $0..$60, the xlat instruction returns the value $0..$60, effectively leaving AL unchanged. However, if AL contains a value in the range $61..$7A (the ASCII codes for a..z), then the xlat instruction replaces the value in AL with a value in the range $41.$5A. The values $41.$5A just happen to be the ASCII codes for A..Z. Therefore, if AL originally contains a lowercase character ($61.$7A), the xlat instruction replaces the value in AL with a corresponding value in the range $61..$7A, effectively converting the original lowercase character ($61..$7A) to an uppercase character ($41.$5A). The remaining entries in the table, like entries $0..$60, simply contain the index into the table of their particular element. Therefore, if AL originally contains a value in the range $7A..$FF, the xlat instruction will return the corresponding table entry that also contains $7A..$FF. As the complexity of the function increases, the performance benefits of the table lookup method increase dramatically. While you would almost never use a lookup table to convert lowercase to uppercase, consider what happens if you want to swap cases, for example, via computation:
That is, it uses the current value of the AL register as an index into the array whose base address is found in EBX. It fetches the byte at that index in the array and copies that byte into the AL register. Intel calls this instruction translate because programmers typically use it to translate characters from one form to another using a lookup table. That's exactly how we are using it here. In the previous example, CnvrtLower is a 256-byte table that contains the values 0..$60 at indices 0..$60, $41.$5A at indices $61.$7A, and $7B..$FF at indices $7Bh.OFF. Therefore, if AL contains a value in the range $0..$60, the xlat instruction returns the value $0..$60, effectively leaving AL unchanged. However, if AL contains a value in the range $61..$7A (the ASCII codes for a..z), then the xlat instruction replaces the value in AL with a value in the range $41.$5A. The values $41.$5A just happen to be the ASCII codes for A..Z. Therefore, if AL originally contains a lowercase character ($61.$7A), the xlat instruction replaces the value in AL with a corresponding value in the range $61..$7A, effectively converting the original lowercase character ($61..$7A) to an uppercase character ($41.$5A). The remaining entries in the table, like entries $0..$60, simply contain the index into the table of their particular element. Therefore, if AL originally contains a value in the range $7A..$FF, the xlat instruction will return the corresponding table entry that also contains $7A..$FF. As the complexity of the function increases, the performance benefits of the table lookup method increase dramatically. While you would almost never use a lookup table to convert lowercase to uppercase, consider what happens if you want to swap cases, for example, via computation:
Computer Networking: A Top-Down Approach (7th Edition)
7th Edition
ISBN:9780133594140
Author:James Kurose, Keith Ross
Publisher:James Kurose, Keith Ross
Chapter1: Computer Networks And The Internet
Section: Chapter Questions
Problem R1RQ: What is the difference between a host and an end system? List several different types of end...
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coding/program
Transcribed Image Text:That is, it uses the current value of the AL register as an index into the
array whose base address is found in EBX. It fetches the byte at that index in
the array and copies that byte into the AL register. Intel calls this instruction
translate because programmers typically use it to translate characters from one
form to another using a lookup table. That's exactly how we are using it here.
In the previous example, CnvrtLower is a 256-byte table that contains the
values 0..$60 at indices 0..$60, $41..$5A at indices $61..$7A, and $7B..$FF at
indices $7Bh..0FF. Therefore, if AL contains a value in the range $0..$60, the
xlat instruction returns the value $0..$60, effectively leaving AL unchanged.
However, if AL contains a value in the range $61.$7A (the ASCII codes for
a..z), then the xlat instruction replaces the value in AL with a value in the
range $41.$5A. The values $41.$5A just happen to be the ASCII codes for
A..Z. Therefore, if AL originally contains a lowercase character ($61.$7A),
the xlat instruction replaces the value in AL with a corresponding value in
the range $61..$7A, effectively converting the original lowercase character
($61..$7A) to an uppercase character ($41.$5A). The remaining entries in
the table, like entries $0..$60, simply contain the index into the table of their
particular element. Therefore, if AL originally contains a value in the range
$7A.$FF, the xlat instruction will return the corresponding table entry that
also contains $7A..$FF.
pter 8
As the complexity of the function increases, the performance benefits of
the table lookup method increase dramatically. While you would almost never
use a lookup table to convert lowercase to uppercase, consider what happens
if you want to swap cases, for example, via computation:
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