May June 1980 - Commodore Computers
May June 1980 - Commodore Computers
May June 1980 - Commodore Computers
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MAY/JUNE. <strong>1980</strong>. 1CW 104 COMPUTE. I 1960. ISSUE 4.<br />
Block Access Method<br />
Block Access Method<br />
Map for a <strong>Commodore</strong><br />
2O4O 2040 Disk Drive<br />
Map for a<br />
San Jose, Calif. <strong>Commodore</strong>anJOSe, f. 95136<br />
OVERVIEW<br />
OVERVIEW<br />
The Block Access Method (BAM) map program will<br />
allow you to see where your files arc are allocated. You<br />
can save and delete files and observe the allocation<br />
technique.<br />
DESCRIPTION<br />
The purpose of the BAM is to protect allocated<br />
files so they are not written over and therefore<br />
destroyed. The BAM map resides on the directory<br />
track 18. The BAM is in the first half of sector 0. O.<br />
The layout looks like this:<br />
BAM Dump<br />
TRACK 18 SECTO R 0<br />
TRACK 18 SECTOR 0<br />
Track<br />
a<br />
b<br />
NUMBER<br />
00 :/112011012 0011 00|l5FFFFlFl<br />
5 I FI<br />
08 : IS 15FFFF1F<br />
FF I F IS 15FFFF1F<br />
IF<br />
10: 15 15FFFF1F FFFF IF 15FFFF1F<br />
IF<br />
18: 15FFFFIFI5 15FFFF1F 15FFFF1F FFFFIF<br />
20: IS 15FFFF FF FF I IF F IS 15FFFF1F FF FF IF<br />
28 : IS 15FFFF1F FF FF I F IS 15FFFF1F FF FF IF<br />
30 : IS 15FFFF1F FF I F IS 15FFFF1F FF FF IF<br />
38 : IS 15FFFF1F FF FF I F IS 15FFFF1F<br />
IF<br />
40: 15 15FFFF1F FFFF IF IS 15FFFF1F<br />
IF<br />
48 : 12 FC FF OF 14 FF FF OF<br />
48 12FCFF0F 14FFFF0F<br />
50 : 14FFFFOF14FFFF0F<br />
58 : 14 14FFFF0F14FFFFOF<br />
60<br />
60<br />
: 14 FF FF OF .............<br />
12 FF FF 03<br />
12FFFFO3<br />
68: 12 12FFFF03 FF03 12 12FFFF03<br />
FF03<br />
70 70: : 12 12FFFFO3 03 12 12FFFFO3<br />
03<br />
78 78: : 12 12FFFF03 FF FF03 ..........._—<br />
II FFFFOI<br />
2 3<br />
4 5<br />
6 7<br />
8 9<br />
10 II<br />
10 11<br />
12 13<br />
14155<br />
1617<br />
16 17<br />
18 19<br />
18 19<br />
2021<br />
21<br />
2223<br />
23<br />
24<br />
25<br />
26 27<br />
28 29<br />
30<br />
11FFFF01 31<br />
80: : II 11 FFFF01 OIII 11 FFFF01 OI<br />
3233<br />
33<br />
82 82: : I11I FFFF01 0 I II 11FFFF01 0 I<br />
3435<br />
35<br />
a •- Address of the next sector which is where the directory begins.<br />
b .- The start of the BAM map for trackk I. 1.<br />
24<br />
30<br />
25<br />
31<br />
Detail Explanation<br />
a b c d<br />
00: : 1201 1 01 0 1 00ll5|FF|FFllF|<br />
001151 FFI FFII FI<br />
a - Total free sectors for track 1. I. In this case it is hex<br />
15 or decimal 21. 2 Since track 1 has a maximum of 21<br />
sectors, track 1 is totally (Otall y empty.<br />
b - The bit configurations for sectors 0 thru Ihru 7. Bit on<br />
means empty sector scc lO r and bit off orf means allocated sector.<br />
hex FF = bits" "1111 III 1111"<br />
"<br />
I Oth Ath sector<br />
lth 1 sector<br />
2th sector<br />
r<br />
3th sector<br />
lOr<br />
4th sector<br />
5th sector<br />
6th sector<br />
7th sector<br />
hex FF = bits ""1111 1111" "<br />
Detail Explanation<br />
7th sector<br />
Therefore all sectors are empty.<br />
c - bit configurations for sectors 8 thru 15<br />
I ~th<br />
8th sector<br />
9th sector<br />
Oth sector<br />
11 llth<br />
sector<br />
12th sector<br />
13th sector<br />
4th sector<br />
15th sector<br />
d - bit configurations for sectors 16 thru 20.<br />
hex IF = bits ""0001 1)1"<br />
1111"<br />
16th sector<br />
7th sector<br />
18th sector<br />
19th sector<br />
20th sector<br />
71 6T?~aq~~aa~<br />
secto r<br />
15th sector<br />
d - bit configurat ions for sectors 16 thru 20.<br />
16th sector<br />
20th sector<br />
In a ny empty disk, the ' d ' byte changes from 1 F, OF , 03, 01<br />
In any empty disk, the 'd' byte changes from IF, OF, 03, 01<br />
to compensate for varying number of sectors per track.<br />
track,<br />
hex 1 IFF is the pattern where there arc 211 sectors<br />
as in tracks 1 thru 17. .<br />
hex OF is the pattern where there are 20 sectors as<br />
in tracks 18 thru 24.<br />
hex I IFF is the pattern where there are arc 18 sectors as in<br />
tracks 25 thru 30. .<br />
hex 1 IFF is the pattern where there are 17 sectors as in<br />
tracks 3311 thru 35.