I did a thing.

I wrote a BASIC program which will create 68 files named “0.TXT” to “67.TXT”. Each file is 2304 bytes so it takes up a full granule. (That is not really important. It just helps makes things obvious if you look at the disk with a hex editor and want to know which file is at which sector.)

After making these files, it uses code from some of my other examples to scan through the directory and display it (FILEGRAN.BAS code, shown later in this post) and then it scans the directory and prints which granule each 1-gran file is using.

I can start with a freshly formatted disk then run this program and see where RS-DOS put each file.

Will it match the order RS-DOS used when making one huge file that takes up all 68 grans? Let’s find out…

10 '68FILES.BAS
20 PRINT "RUN THIS ON A BLANK DISK."
30 INPUT "DRIVE #";DR
40 'SWITCH TO THAT DRIVE
50 DRIVE DR
60 'GOTO 140
70 'MAKE FILES 0-67
80 FOR G=0 TO 67
90 F$=MID$(STR$(G),2)+".TXT"
100 PRINT "MAKING ";F$;
110 OPEN "O",#1,F$
120 CLOSE #1:PRINT
130 NEXT
140 'FILEGRAN.BAS
150 'DIR WITHOUT FILE SIZES
160 CLEAR 512:DIM SP$(1)
170 ' S - SECTOR NUMBER
180 FOR S=3 TO 11
190 ' SP$(0-1) - SECTOR PARTS
200 DSKI$ DR,17,S,SP$(0),SP$(1)
210 ' P - PART OF SECTOR
220 FOR P=0 TO 1
230 ' E - DIR ENTRY (4 P/SECT.)
240 FOR E=0 TO 3
250 ' GET 32 BYTE DIR ENTRY
260 DE$=MID$(SP$(P),1+E*32,32)
270 ' FB - FIRST BYTE OF NAME
280 FB=ASC(LEFT$(DE$,1))
290 ' SKIP DELETED FILES
300 IF FB=0 THEN 440
310 ' WHEN 255, DIR IS DONE
320 IF FB=255 THEN 470
330 ' PRINT NAME AND EXT.
340 'PRINT LEFT$(DE$,8);TAB(9);MID$(DE$,9,3);
350 ' FIRST TWO CHARS ONLY
360 PRINT LEFT$(DE$,2);"-";
361 'PRINT #-2,LEFT$(DE$,2);",";
370 ' FILE TYPE
380 'PRINT TAB(13);ASC(MID$(DE$,12,1));
390 ' BINARY OR ASCII
400 'IF ASC(MID$(DE$,13,1))=0 THEN PRINT "B"; ELSE PRINT "A";
410 ' STARTING GRANULE
420 PRINT USING("##  ");ASC(MID$(DE$,14,1));
421 'PRINT #-2,ASC(MID$(DE$,14,1))
430 CL=CL+1:IF CL=5 THEN CL=0:PRINT
440 NEXT
450 NEXT
460 NEXT
470 END

I modified this program to output to the printer (PRINT #-2) and then capture that output in the Xroar emulator in a text file. That gave me data which I put in a spreadsheet.

 68 Files
FILE  GRAN
0     32
1     33
2     34
3     35
4     30
5     31
6     36
7     37
8     28
9     29
10    38
11    39
12    26
13    27
14    40
15    41
16    24
17    25
18    42
19    43
20    22
21    23
22    44
23    45
24    20
25    21
26    46
27    47
28    18
29    19
30    48
31    49
32    16
33    17
34    50
35    51
36    14
37    15
38    52
39    53
40    12
41    13
42    54
43    55
44    10
45    11
46    56
47    57
48     8
49     9
50    58
51    59
52     6
53     7
54    60
55    61
56     4
57     5
58    62
59    63
60     2
61     3
62    64
63    65
64     0
65     1
66    66
67    67  

Next, I used a second program on a freshly formatted disk to create one big file fully filling up the disk. (The very last PRINT to the file will create a ?DF ERROR, which I now think is a bug. It should not do that until I try to write the next byte, I think.)

10 '1BIGFILE.BAS
20 PRINT"RUN THIS ON A BLANK DISK."
30 INPUT "DRIVE #";DR
40 'SWITCH TO THAT DRIVE
50 DRIVE DR
60 'MAKE ONE BIG 68 GRAN FILE
70 OPEN "O",#1,"1BIGFILE.TXT"
80 FOR G=0 TO 67
90 PRINT G;
100 T$=STRING$(128,G)
110 FOR T=1 TO 18
120 PRINT ".";
130 PRINT #1,T$;
140 NEXT
150 PRINT
160 NEXT
170 CLOSE #1
180 END

I ran another test program which would read the directory, then print out the granule chain of each file on the disk.

10 ' FILEGRAN.BAS
20 '
30 ' 0.0 2025-11-20 BASED ON FILEINFO.BAS
40 '
50 ' E$(0-1) - SECTOR HALVES
60 ' FT$ - FILE TYPE STRINGS
70 '
80 CLEAR 1500:DIM E$(1),FT$(3)
90 FT$(0)="BPRG":FT$(1)="BDAT":FT$(2)="M/L ":FT$(3)="TEXT "
100 '
110 ' DIR HOLDS UP TO 72 ENTRIES
120 '
130 ' NM$ - NAME
140 ' EX$ - EXTENSION
150 ' FT - FILE TYPE (0-3)
160 ' AF - ASCII FLAG (0/255)
170 ' FG - FIRST GRANULE #
180 ' BU - BYTES USED IN LAST SECTOR
190 ' SZ - FILE SIZE
200 ' GM - GRANULE MAP
210 '
220 DIM NM$(71),EX$(71),FT(71),AF(71),FG(71),BU(71),SZ(71),GM(67)
230 '
240 INPUT "DRIVE";DR
250 '
260 ' FILE ALLOCATION TABLE
270 ' 68 GRANULE ENTRIES
280 '
290 DIM FA(67)
300 DSKI$ DR,17,2,G$,Z$:Z$=""
310 FOR G=0 TO 67
320 FA(G)=ASC(MID$(G$,G+1,1))
330 NEXT
340 '
350 ' READ DIRECTORY
360 '
370 DE=0
380 FOR S=3 TO 11
390 DSKI$ DR,17,S,E$(0),E$(1)
400 '
410 ' PART OF SECTOR
420 '
430 FOR P=0 TO 1
440 '
450 ' ENTRY WITHIN SECTOR PART
460 '
470 FOR E=0 TO 3
480 '
490 ' DIR ENTRY IS 32 BYTES
500 '
510 E$=MID$(E$(P),E*32+1,32)
520 '
530 ' NAME IS FIRST 8 BYTES
540 '
550 NM$(DE)=LEFT$(E$,8)
560 '
570 ' EXTENSION IS BYTES 9-11
580 '
590 EX$(DE)=MID$(E$,9,3)
600 '
610 ' FILE TYPE IS BYTE 12
620 '
630 FT(DE)=ASC(MID$(E$,12,1))
640 '
650 ' ASCII FLAG IS BYTE 13
660 '
670 AF(DE)=ASC(MID$(E$,13,1))
680 '
690 ' FIRST GRANUAL IS BYTE 14
700 '
710 FG(DE)=ASC(MID$(E$,14,1))
720 '
730 ' BYTES USED IN LAST SECTOR
740 ' ARE IN BYTES 15-16
750 '
760 BU(DE)=ASC(MID$(E$,15,1))*256+ASC(MID$(E$,16,1))
770 '
780 ' IF FIRST BYTE IS 255, END
790 ' OF USED DIR ENTRIES
800 '
810 IF LEFT$(NM$(DE),1)=CHR$(255) THEN 1500
820 '
830 ' IF FIRST BYTE IS 0, FILE
840 ' WAS DELETED
850 '
860 IF LEFT$(NM$(DE),1)=CHR$(0) THEN 1480
870 '
880 ' SHOW DIRECTORY ENTRY
890 '
900 PRINT NM$(DE);TAB(9);EX$(DE);"  ";FT$(FT(DE));" ";
910 IF AF(DE)=0 THEN PRINT"BIN"; ELSE PRINT "ASC";
920 '
930 ' CALCULATE FILE SIZE
940 ' SZ - TEMP SIZE
950 ' GN - TEMP GRANULE NUM
960 ' SG - SECTORS IN LAST GRAN
970 ' GC - GRANULE COUNT
980 '
990 SZ=0:GN=FG(DE):SG=0:GC=0
1000 '
1010 ' GET GRANULE VALUE
1020 ' GV - GRAN VALUE
1030 '
1040 GV=FA(GN):GM(GC)=GN:GC=GC+1
1050 '
1060 ' IF TOP TWO BITS SET (C0
1070 ' OR GREATER), IT IS THE
1080 ' LAST GRANULE OF THE FILE
1090 ' SG - SECTORS IN GRANULE
1100 '
1110 IF GV>=&HC0 THEN SG=(GV AND &H1F):GOTO 1280
1120 '
1130 ' IF NOT, MORE GRANS
1140 ' ADD GRANULE SIZE
1150 '
1160 SZ=SZ+2304
1170 '
1180 ' MOVE ON TO NEXT GRANULE
1190 '
1200 GN=GV
1210 GOTO 1040
1220 '
1230 ' DONE WITH GRANS
1240 ' CALCULATE SIZE
1250 '
1260 ' FOR EMPTY FILES
1270 '
1280 IF SG>0 THEN SG=SG-1
1290 '
1300 ' FILE SIZE IS SZ PLUS
1310 ' 256 BYTES PER SECTOR
1320 ' IN LAST GRAN PLUS
1330 ' NUM BYTES IN LAST SECT
1340 '
1350 SZ(DE)=SZ+(SG*256)+BU(DE)
1360 PRINT " ";SZ(DE)
1370 '
1380 ' SHOW GRANULE MAP
1390 '
1400 C=0:PRINT "  ";
1410 FOR I=0 TO GC-1
1420 PRINT USING"##";GM(I);
1430 C=C+1:IF C=10 THEN PRINT:PRINT "  ";:C=0 ELSE PRINT " ";
1440 NEXT:PRINT
1450 '
1460 ' INCREMENT DIR ENTRY
1470 '
1480 DE=DE+1
1490 NEXT:NEXT:NEXT
1500 END
1510 ' SUBETHASOFTWARE.COM

Since there is only one big file on this disk, fully filling it, it only has one 68-entry granule chain to print. I modified the code to PRINT#-2 these values to the virtual printer so I could then copy the numbers into the same spreadsheet:

 68 Files  Big File
FILE  GRAN  GRAN
0     32    32
1     33    33
2     34    34
3     35    35
4     30    36
5     31    37
6     36    38
7     37    39
8     28    40
9     29    41
10    38    42
11    39    43
12    26    44
13    27    45
14    40    46
15    41    47
16    24    48
17    25    49
18    42    50
19    43    51
20    22    52
21    23    53
22    44    54
23    45    55
24    20    56
25    21    57
26    46    58
27    47    59
28    18    60
29    19    61
30    48    62
31    49    63
32    16    64
33    17    65
34    50    66
35    51    67
36    14    30
37    15    31
38    52    28
39    53    29
40    12    26
41    13    27
42    54    24
43    55    25
44    10    22
45    11    23
46    56    20
47    57    21
48     8    18
49     9    19
50    58    16
51    59    17
52     6    14
53     7    15
54    60    12
55    61    13
56     4    10
57     5    11
58    62    8
59    63    9
60     2    6
61     3    7
62    64    4
63    65    5
64     0    2
65     1    3
66    66    0
67    67    1

Now it seems clearly obvious that RS-DOS does something different when making a new file, versus what it does when expanding an existing file into a new granule.

I wanted a way to visualize this so, of course, I wrote a program to help me create a full ASCII representation of the granules, then edited the rest by hand.

                           68    1 Big
                           Files File
Track  0 +------------+
         | Granule  0 |    64    66
         | Granule  1 |    65    67
Track  1 +------------+
         | Granule  2 |    60    64
         | Granule  3 |    61    65
Track  2 +------------+
         | Granule  4 |    56    62
         | Granule  5 |    57    63
Track  3 +------------+
         | Granule  6 |    52    60
         | Granule  7 |    53    61
Track  4 +------------+
         | Granule  8 |    48    58
         | Granule  9 |    49    59
Track  5 +------------+
         | Granule 10 |    44    56
         | Granule 11 |    45    57
Track  6 +------------+
         | Granule 12 |    40    54
         | Granule 13 |    41    55
Track  7 +------------+
         | Granule 14 |    36    52
         | Granule 15 |    37    53
Track  8 +------------+
         | Granule 16 |    32    50
         | Granule 17 |    33    51
Track  9 +------------+
         | Granule 18 |    28    48
         | Granule 19 |    29    49
Track 10 +------------+
         | Granule 20 |    24    46
         | Granule 21 |    25    47
Track 11 +------------+
         | Granule 22 |    20    44
         | Granule 23 |    21    45
Track 12 +------------+
         | Granule 24 |    16    42
         | Granule 25 |    17    43
Track 13 +------------+
         | Granule 26 |    12    40
         | Granule 27 |    13    41
Track 14 +------------+
         | Granule 28 |    8     38
         | Granule 29 |    9     39
Track 15 +------------+
         | Granule 30 |    4     36
         | Granule 31 |    5     37
Track 16 +------------+
         | Granule 32 |    0     0 <- both start the same
         | Granule 33 |    1     1
Track 17 +------------+
         | FAT &      |
         | Directory  |
Track 18 +------------+
         | Granule 34 |    2     2
         | Granule 35 |    3     3
Track 19 +------------+
         | Granule 36 |    6     4 <- then big file continues
         | Granule 37 |    7     5    writing to the end
Track 20 +------------+
         | Granule 38 |    10    6
         | Granule 39 |    11    7
Track 21 +------------+
         | Granule 40 |    14    8
         | Granule 41 |    15    9
Track 22 +------------+
         | Granule 42 |    18    10
         | Granule 43 |    19    11
Track 23 +------------+
         | Granule 44 |    22    12
         | Granule 45 |    23    13
Track 24 +------------+
         | Granule 46 |    26    14
         | Granule 47 |    27    15
Track 25 +------------+
         | Granule 48 |    30    16
         | Granule 49 |    31    17
Track 26 +------------+
         | Granule 50 |    34    18
         | Granule 51 |    35    19
Track 27 +------------+
         | Granule 52 |    38    20
         | Granule 53 |    39    21
Track 28 +------------+
         | Granule 54 |    42    22
         | Granule 55 |    43    23
Track 29 +------------+
         | Granule 56 |    46    24
         | Granule 57 |    47    25
Track 30 +------------+
         | Granule 58 |    50    26
         | Granule 59 |    51    27
Track 31 +------------+
         | Granule 60 |    54    28
         | Granule 61 |    55    29
Track 32 +------------+
         | Granule 62 |    58    30
         | Granule 63 |    59    31
Track 33 +------------+
         | Granule 64 |    62    32
         | Granule 65 |    63    33
Track 34 +------------+
         | Granule 66 |    66    34
         | Granule 67 |    67    35 <- then big file continues
         +------------+                at Track 16

Interesting! For small files, it alternates tracks starting before Track 17 (FAT/Directory) then after, repeating. For a big file, it starts like that before Track 17, then after and continues to the end of Track 35, then goes before Track 17 and works back to the start of the disk.

Do I understand the sequence correctly?

To be continued…

See also: part 1, part 2 and part 3.

A correction, and discovering the order RS-DOS writes things…

A correction from part 2… This example program had “BIN” and “ASC” mixed up. 0 should represent BINary files, and 255 for ASCii files. I fixed it in line 920. (I will try to edit/fix the original post when I get a moment.)

10 ' FILEINFO.BAS
20 '
30 ' 0.0 2023-01-25 ALLENH
40 ' 0.1 2023-01-26 ADD DR
50 ' 0.2 2023-01-27 MORE COMMENTS
55 ' 0.3 2025-11-18 BIN/ASC FIX
60 '
70 ' E$(0-1) - SECTOR HALVES
80 ' FT$ - FILE TYPE STRINGS
90 '
100 CLEAR 1500:DIM E$(1),FT$(3)
110 FT$(0)="BPRG":FT$(1)="BDAT":FT$(2)="M/L ":FT$(3)="TEXT "
120 '
130 ' DIR HOLDS UP TO 72 ENTRIES
140 '
150 ' NM$ - NAME
160 ' EX$ - EXTENSION
170 ' FT - FILE TYPE (0-3)
180 ' AF - ASCII FLAG (0/255)
190 ' FG - FIRST GRANULE #
200 ' BU - BYTES USED IN LAST SECTOR
210 ' SZ - FILE SIZE
220 '
230 DIM NM$(71),EX$(71),FT(71),AF(71),FG(71),BU(71),SZ(71)
240 '
250 INPUT "DRIVE";DR
260 '
270 ' FILE ALLOCATION TABLE
280 ' 68 GRANULE ENTRIES
290 '
300 DIM FA(67)
310 DSKI$ DR,17,2,G$,Z$:Z$=""
320 FOR G=0 TO 67
330 FA(G)=ASC(MID$(G$,G+1,1))
340 NEXT
350 '
360 ' READ DIRECTORY
370 '
380 DE=0
390 FOR S=3 TO 11
400 DSKI$ DR,17,S,E$(0),E$(1)
410 '
420 ' PART OF SECTOR
430 '
440 FOR P=0 TO 1
450 '
460 ' ENTRY WITHIN SECTOR PART
470 '
480 FOR E=0 TO 3
490 '
500 ' DIR ENTRY IS 32 BYTES
510 '
520 E$=MID$(E$(P),E*32+1,32)
530 '
540 ' NAME IS FIRST 8 BYTES
550 '
560 NM$(DE)=LEFT$(E$,8)
570 '
580 ' EXTENSION IS BYTES 9-11
590 '
600 EX$(DE)=MID$(E$,9,3)
610 '
620 ' FILE TYPE IS BYTE 12
630 '
640 FT(DE)=ASC(MID$(E$,12,1))
650 '
660 ' ASCII FLAG IS BYTE 13
670 '
680 AF(DE)=ASC(MID$(E$,13,1))
690 '
700 ' FIRST GRANUAL IS BYTE 14
710 '
720 FG(DE)=ASC(MID$(E$,14,1))
730 '
740 ' BYTES USED IN LAST SECTOR
750 ' ARE IN BYTES 15-16
760 '
770 BU(DE)=ASC(MID$(E$,15,1))*256+ASC(MID$(E$,16,1))
780 '
790 ' IF FIRST BYTE IS 255, END
800 ' OF USED DIR ENTRIES
810 '
820 IF LEFT$(NM$(DE),1)=CHR$(255) THEN 1390
830 '
840 ' IF FIRST BYTE IS 0, FILE
850 ' WAS DELETED
860 '
870 IF LEFT$(NM$(DE),1)=CHR$(0) THEN 1370
880 '
890 ' SHOW DIRECTORY ENTRY
900 '
910 PRINT NM$(DE);TAB(9);EX$(DE);"  ";FT$(FT(DE));" ";
920 IF AF(DE)=0 THEN PRINT"BIN"; ELSE PRINT "ASC";
930 '
940 ' CALCULATE FILE SIZE
950 ' SZ - TEMP SIZE
960 ' GN - TEMP GRANULE NUM
970 ' SG - SECTORS IN LAST GRAN
980 '
990 SZ=0:GN=FG(DE):SG=0
1000 '
1010 ' GET GRANULE VALUE
1020 ' GV - GRAN VALUE
1030 '
1040 GV=FA(GN)
1050 '
1060 ' IF TOP TWO BITS SET (C0
1070 ' OR GREATER), IT IS THE
1080 ' LAST GRANULE OF THE FILE
1090 ' SG - SECTORS IN GRANULE
1100 '
1110 IF GV>=&HC0 THEN SG=(GV AND &H1F):GOTO 1280
1120 '
1130 ' ELSE, MORE GRANS
1140 ' ADD GRANULE SIZE
1150 '
1160 SZ=SZ+2304
1170 '
1180 ' MOVE ON TO NEXT GRANULE
1190 '
1200 GN=GV
1210 GOTO 1040
1220 '
1230 ' DONE WITH GRANS
1240 ' CALCULATE SIZE
1250 '
1260 ' FOR EMPTY FILES
1270 '
1280 IF SG>0 THEN SG=SG-1
1290 '
1300 ' FILE SIZE IS SZ PLUS
1310 ' 256 BYTES PER SECTOR
1320 ' IN LAST GRAN PLUS
1330 ' NUM BYTES IN LAST SECT
1340 '
1350 SZ(DE)=SZ+(SG*256)+BU(DE)
1360 PRINT " ";SZ(DE)
1370 DE=DE+1
1380 NEXT:NEXT:NEXT
1390 END
1400 ' SUBETHASOFTWARE.COM

To test this routine, I created a program that let me type a file size (in bytes) and then it would make a .TXT file with that size as the filename (i.e, for 3000 bytes, it makes “3000.TXT”) and then I could run it through this program and see if everything matched.

It opens a file with the size as the filename, then writes out “*” characters to fill the file. This will be painfully slow for large files. If you want to make it much faster, share your work in a comment.

10 ' MAKEFILE.BAS
20 '
30 ' 0.0 2025-11-18 ALLENH
40 '
50 INPUT "FILE SIZE";SZ
60 F$=MID$(STR$(SZ),2)+".TXT"
70 OPEN "O",#1,F$
80 FOR A=1 TO SZ:PRINT #1,"*";:NEXT
90 CLOSE #1
100 DIR
110 GOTO 50
120 ' SUBETHASOFTWARE.COM

I was able to use this program in the Xroar emulator to create files of known sizes so I could verify the FILEINFO.BAS program was doing the proper thing.

It seems to be, so let’s move on…

A funny thing happened on the way to the disk…

I have been digging in to disk formats (OS-9 and RS-DOS) lately, and learning more things I wish I knew “back in the day.” For instance, I was curious how RS-DOS allocates granules (see part 1) when adding files to the disk. I wrote a test program that would write out 2304-byte blocks of data (the size of a granule) full of the number of the block. i.e., for the first write, I’d write 2304 0’s, then 2304 1’s and so on. My simple program looks like this:

10 'GRANULES.BAS
20 OPEN "O",#1,"GRANULES.TXT"
30 FOR G=0 TO 67
40 PRINT G;
50 T$=STRING$(128,G)
60 FOR T=1 TO 18
65 PRINT ".";
70 PRINT #1,T$;
80 NEXT
90 PRINT
100 NEXT
110 CLOSE #1

I ran this on a freshly formatted disk and let it fill the whole thing up. The very last write errors with a ?DF ERROR (disk full) so it never makes it to the close. I guess you can’t write that last byte without an error?

Now I should be able to look a the bytes on the disk and see where the 0’s went, the 15’s went, and so on, and see the order RS-DOS allocated those granules.

I made a simple test program for this:

0 'GRANDUMP.BAS
10 CLEAR 512
20 FOR G=0 TO 67
30 T=INT((G)/2):IF T>16 THEN T=T+1
40 IF INT(G/2)*2=G THEN S1=10:S2=18 ELSE S1=1:S2=9
50 'PRINT "GRANULE";G;TAB(13);"T";T;TAB(20);"S";S1;"-";S2
54 DSKI$0,T,S1,A$,B$
55 PRINT "GRANULE";G;ASC(A$)
60 NEXT G

Ignore the commented out stuff. Initially I was just getting it to convert a granule to Track/Sectors with code to skip Track 17 (FAT/Directory). And, to be honest, I had an AI write this and I just modified it ;-)

I then modified it to PRINT#-2 to the printer, and ran it in Xroar with the printer redirected to a text file. That gave me the following output:

GRANULE 0  67
GRANULE 1  66
GRANULE 2  65
GRANULE 3  64
GRANULE 4  63
GRANULE 5  62
GRANULE 6  61
GRANULE 7  60
GRANULE 8  59
GRANULE 9  58
GRANULE 10  57
GRANULE 11  56
GRANULE 12  55
GRANULE 13  54
GRANULE 14  53
GRANULE 15  52
GRANULE 16  51
GRANULE 17  50
GRANULE 18  49
GRANULE 19  48
GRANULE 20  47
GRANULE 21  46
GRANULE 22  45
GRANULE 23  44
GRANULE 24  43
GRANULE 25  42
GRANULE 26  41
GRANULE 27  40
GRANULE 28  39
GRANULE 29  38
GRANULE 30  37
GRANULE 31  36
GRANULE 32  1 
GRANULE 33  0 
GRANULE 34  3 
GRANULE 35  2 
GRANULE 36  5 
GRANULE 37  4 
GRANULE 38  7 
GRANULE 39  6 
GRANULE 40  9 
GRANULE 41  8 
GRANULE 42  11
GRANULE 43  10
GRANULE 44  13
GRANULE 45  12
GRANULE 46  15
GRANULE 47  14
GRANULE 48  17
GRANULE 49  16
GRANULE 50  19
GRANULE 51  18
GRANULE 52  21
GRANULE 53  20
GRANULE 54  23
GRANULE 55  22
GRANULE 56  25
GRANULE 57  24
GRANULE 58  27
GRANULE 59  26
GRANULE 60  29
GRANULE 61  28
GRANULE 62  31
GRANULE 63  30
GRANULE 64  33
GRANULE 65  32
GRANULE 66  35
GRANULE 67  34

Now I can see the order that RS-DOS allocates data on an empty disk.

The number in the third column represents the value of the bytes written to that 2304 granule. When I see “GRANULE 67” contains “34” as data, I know it was the 35th (numbers 0-34) granule written out.

Granules 0-33 are on tracks 0-16, then track 17 is skipped, then the remaining granules 34-67 are on tracks 18-34.

You can see that RS-DOS initially writes the data close to track 17, reducing the time it takes to seek from the directory to the file data. This makes sense, though as a teen, I guess I had some early signs of O.C.D. because I thought the directory should be at the start of the disk, and not in the middle ;-)

I brought this data into a spreadsheet, then sorted it by the “data” value (column 3). This let me see the order that granules are allocated (written to). I will add some comments:

GRANULE	33	0 <- first went to gran 33
GRANULE	32	1 <- second went to gran 32

...then it starts writing after Track 17...

GRANULE	35	2 <- third went to gran 35
GRANULE	34	3 <- fourth went to gran 34
GRANULE	37	4
GRANULE	36	5
GRANULE	39	6
GRANULE	38	7
GRANULE	41	8
GRANULE	40	9
GRANULE	43	10
GRANULE	42	11
GRANULE	45	12
GRANULE	44	13
GRANULE	47	14
GRANULE	46	15
GRANULE	49	16
GRANULE	48	17
GRANULE	51	18
GRANULE	50	19
GRANULE	53	20
GRANULE	52	21
GRANULE	55	22
GRANULE	54	23
GRANULE	57	24
GRANULE	56	25
GRANULE	59	26
GRANULE	58	27
GRANULE	61	28
GRANULE	60	29
GRANULE	63	30
GRANULE	62	31
GRANULE	65	32
GRANULE	64	33
GRANULE	67	34
GRANULE	66	35

...now that it has written to the final Track 35 (gran 66-67)...

GRANULE	31	36 <- before Track 17 and the original writes.
GRANULE	30	37
GRANULE	29	38
GRANULE	28	39
GRANULE	27	40
GRANULE	26	41
GRANULE	25	42
GRANULE	24	43
GRANULE	23	44
GRANULE	22	45
GRANULE	21	46
GRANULE	20	47
GRANULE	19	48
GRANULE	18	49
GRANULE	17	50
GRANULE	16	51
GRANULE	15	52
GRANULE	14	53
GRANULE	13	54
GRANULE	12	55
GRANULE	11	56
GRANULE	10	57
GRANULE	9	58
GRANULE	8	59
GRANULE	7	60
GRANULE	6	61
GRANULE	5	62
GRANULE	4	63
GRANULE	3	64
GRANULE	2	65
GRANULE	1	66
GRANULE	0	67 <- last write at the very first gran

And down the rabbit hole I go. Again. I have tasked an A.I. with creating some simple scripts to manipulate RS-DOS disk images (just for fun; the toolshed “decb” command already exists and works great and does more). While I understood the basic structure for an RS-DOS disk, I did not understand “how” RS-DOS actually allocated those granules. Now I have some insight. Perhaps I can make my tools replicate writing in the same way that RS-DOS itself does.

Look for a part 4. I have some more experiments to share.

To be continued…