The Value Of Index Files; Chapter 7: Direct Access Commands; Diskette Organization - Commodore 1541 User Manual

3030 GOSUB 59990
3040 INPUT# l ,NA$,SA$,CS$,ZP$
3050 GOSUB 59990
3060 PRINT#l5, "P" +CHR$(96+ 3) +
CHR$(RL) + CHR$(RH)
Check for disk errors
Read in fields
Re-position for safety
Here are the Jines needed to read back the version with fixed length fields :
3000 RH= INT(RFJ256)
3010 RL=RE-256*RH
3020 PRINT#l5,"P" +CHR$(96+3) +CHR$
( RL) + CHR$(RH)
3030 GOSUB 59990
3040 INPUT# l ,DA$
3050 GOSUB 59990
3060 PRINT#l5, "P" + CHR$(96+ 3) +
CHR$(RL) + CHR$(RH)
3070 NA$= LEFf$(DA$,27)
3080 SA$= MID$(DA$ ,28 ,27)
3090 CS$= MID$(DA$,55,23)
3100 ZP$ = RIGHT$(DA$, 10)
Read in entire record
Split data into fields
This ends our discussion
of
n·l .111ve files . A complete "RELA
TIY~
FILE" program .
similar to the examples in this
ch.ipt~r.
is included on the Test/Demo diskette .
CHAPTER 7
DIRECT ACCESS COMMANDS
~
fOOL FOR ADVANCED USERS
Direct access commands specify individual sectors on the diskette, reading and
..nung information entirely under your direction . This gives them almost complete
~bility
in data-handling proerams, but also imposes tremendous responsibilities on the
~er, to be sure nothing goes awry. As a result, they are normally used only in
tdl!Plex commercial programs able to properly organize data without help from the disk
,JIVC
itself.
A
far more common use of direct access commands is in utility programs used to
fl(9I
and alter parts of the diskette that are not normally seen directly . For instance . such
,1Jflll1lllnds can be used to change the name of a diskette without erasing all of its
~s,
to lock a program so it can't be erased , or hide your name in a location where it
'°"'t be expected .
DISKETTE ORGANIZATION
There are a total of 683 blocks on a 1541 diskene , of which 664 are available for use,
itb the rest reserved
for
the BAM (Block Availability Map) and the Directory .
The diske!!e's surface is divided into tracks, which are laid out as concentric circles
111
lhe
surface of the diskette . There are 35 different tracks , starting with track I at the
ldlide of the diske!!e to track 35 at the center. Track 18 is used for the directory, and the
DOS
fills up the diskette from the center outward , alternately in both directions.
THE VALUE OF INDEX FILES (ADVANCED USERS) Each track is subdivided into sectors (also called blocks) . Because there is more
. d
1
. fil
room
on the outer tracks, there are more sectors per track there. The outermost tracks
In the last two chapters we have learned how to use sequential an re katlve b
.Cl
1
.mn 21 sectors each , while the innermost ones only have 17 sectors each . The table
ft
d th · th the sequential file used to eep ne
separately. But they are o en use toge . e r• wi . Th wa the
!Jclow
shows the number of sectors per track .
records of which name in the relative file 1s stored m each record number. al .
Y
1
. d ·
tri
array and sorted alphabetical
Y
contents of the sequential file can be rea mto as ng . fi d
an
Table 6.1: Track and Sector Format
After sorting, a technique known as a binary search can be used to
v~ry quic~y·
n file TRACK NUMBER SECTOR NUMBERS TOTAL SECTORS
entered name in the array' and read in or write the associated record m. the. re
~uve
21
Ad vanced programs can maintain two or more such index files, sorted m d1ffenng
ways
I to 17 0 through 20 19
18 to 24 0 through 18
simultaneously. 25 to 30
O
through 17 18
64
31 to 35 0 through 16 17
In
this chapter we will describe the DOS commands for directly reading and writing
:nylrack and block on the diskette, as well as the commands used to mark blocks as used
llllused. Unless otherwise notes, all direct access commands are the same in both Basic
·and Basic 3.5 .
OitNiNG A DATA CHANNEL FOR DIRECT ACCESS
When working with direct access data, you -need two channels open to the disk: the
' d channel we've used throughout the book, and another for data . The command
illne1 is opened with the usual OPEN 15,8,15 or equivalent. A direct access data
65