May June 1980 - Commodore Computers

MAY/JUNE. I 1980. 198O ISSUE 4. COMPUTE. 105 1O5
OBSERVATIONS USING THE BAM MAP
PROGRAM
The BAM turns ofT off the bits when it allocates a
sector. The BAM Map Program looks at these bits
and if the bit is on (meaning it is free and has not
been allocated) it will print either a "~~ "&" : " or a white
sqare. By looking at the map you can determine
how full or empty the disk is.
Varying numbers of sectors.
The reason for the varying number of sectors per
track is LO to pack more data on the disk. Using the
worse case which is 17 sectors per track and propagate
throughout the disk would decrease the number
of sectors per track by 95 sectors or 24k.
Sectors not contiguous
The sectors are in 255 byte blocks. A program
file, which is stored in 255 bytes, is not written
on the disk contiguously but wrillen written approximately
one-halftrack apart. . Using thee BAM program, m , you
can see when youu save a program on a empty disk,
that DOS will save the first 255 bytes on track 17
secLOr sector 0, the second 255 bytes on sector 10, the
third 255 bytes on sector 3, and so on. The purpose
of these gaps is to speed up the processing by not
S0
El
C2
T3
0 4
R5
6
~
to
II
TRACKS TRftCKS 11111111112222222222333333
1234567891312345678901234567891312345
123456785012345678581234567B90I3345
12
13
14
15
16
17
IS 18
19
2Q 20 wsmmmmmmmmt—free blks= s?e
CKS TRACI(S 111111111112222222222333333
U11111112222222222333333
12343678901234567890123456765012345
12345678901234567890123456789012345
se Sfi
I
El
I
C2
II
I
T3
04
• I
I
R5
6
• I
I
7
8 "
I
I
9
18 10
I
I
11 II
12 •
I
I
13
14 I "
I
I
15 I
It 16 I • I
17 I
18 I
,-- 19 I • .--
I
I I DR*=EMPTV
OR*=EMPT'y'
20 I
l"FREE :REE BLKS= £lLKS= @ £I
References:
Parsons, s, James Jamcs C, C .. "Display y Track and d Sector", Scctor", Commodore
Newsletter Ncwslclle!' Vol. 1I Number 8, January 1980.
.
Commodore Business ti ll Machines, Commodore CBM C Dual Floppy
py
Disk Model 2040 User U SCI' Manual, al , July J y 1979.
waiting for a full rotation of the disk. If the pro­
waiting for a full rotation of the disk. If the pro
gram was written contiguously after each write, , DOS
would have to wait an entire rot rotation of the disk to
write the next sequental sector. Thus, when one is
looking g at the BAM Map, one will see where alternating
sectors are allocated.
ing sectors are allocated.
Allocation of sectors
DOS allocates disk space very efficiently. SecLOrs
Sectors
are allocated around the directory (track 18) . This
are allocated around the directory (track 18). This
reduces the read/write head movement because it reads
the directory first, , then reads the fil file. By having the
file e close to the directory, head movement is reduced. .
Where sectors are allocated
When you delete the first program on a full disk,
the BAM Map will show free sectors near the directory.
When you save a new program, it will start by
allocating those free sectors nearest the directory and
will start filling in where you deleted the old file.
If thee new program is larger than the old program,
it will tJ'>' try to (o allocate sectors further and further
from thee directory. By using this allocation technique,
the need for a disk compress is eliminated.
PROGRAM EXPLANATION
PROGRAM EXPLANATION
100-170 Initialization
180-190 Which drive?
200-430 Prints s the BAM Map outline. .
100 REM* REI·l* BLOCK ACCESS METHOD NETHOD DUMP *
110 REM* WRITTEN BY TOM T011 CONRAD *
120 REM*
130 REM* RE~I*
140 REM* INITIALIZATION
I NITIAL IZATION *
150 DIM A(4)
160 NL$=CHR$(0)
170 T=0: REM REI1 TOTAL FREE BLOCKS
175 REM* REI1* WHICH DRIVE *
180 PRINT"fiVHHDRIVE? "fifttWDRIVE?""
190 GET D$: IF D$="" GOTO 190
195 REM* PRINTS THE BAM MAP ~lAP OUTLINE *
200 PRINT"fi rTRACKSr LTRACKSf 111111111122222
-.22222333333"
, 210 PRINT" 123456789012345678901234567
8 -.89012345"
,89012345"
220 PRINT"! PRINT"} $$$SS$$$SS$S$$$$SSSSSSSSSSS
$$$$$$$$$$$$$$$$$$$$$$$$$$$
-,$$$$$$$S"
,$$$$$$$$"
230 PRINT"rSr0_L
"LSf0.'. '
, 1"
240 PRINT"xEfll
".r.Efl.'.
,
-i
1"
250 PRINT"j:Cr21
".r.Cf2.'.
, 1 "
260 PRINT"rTf31
".r.TD.'.
, 1"
270 PRINT"iOr4J.
"Lof4.'. ,
, 1 "
280 PRINTnrRf51
".r.Rf5.'.
, 1 "
290 PRINT" " 61 6.'.
,
-i ■1 "
300 PRINT" 71 7.'.
, 1"