>CoreNotes2 3/2/88

IMPLEMENTING A FILING SYSTEM WITH FILECORE

Introduction

You must provide a parent module that starts up another instantiation of FileCore, handles various high level filing system specific operations and services the requests for low level operations.

FileCore SWIs

These are provided for the use of the parent module. For those marked with a * the caller must prove it's identity by passing in R8 the address of the private word of it's instantiation of FileCore

&40540 DiscOp *

For fuller details on parameters and results see low level below.

entry
R1 bits 0-7 reason code and option bits, bits 8-31 if non zero are bits 2-25 of a word aligned ptr to an alternative disc record R2 disc address
R3 RAM ptr
R4 length
R8 ->FileCore instance private word

exit
R0 IF V set error pointer else preserved R1 preserved
R2-R4 adjusted to reflect amount transferred

&40541 Create

entry
R0 -> descriptor block
R1 -> module base
R2 -> parent modules private word
R3 b0 -b7 number of floppies
b8 -b15 number of winnies
b16-b24 default drive
b25-b31 start up options all 0 except bit 30 set <=> No dir state R4 suggested size for directory cache
R5 suggested number of buffers for file cache 1K + 48 bytes each R6 winnie map sizes, 1 byte for each winnie = map size/256, ie 2 for old map

exit
R0 if error V set R0->error block else R0->FileCore instance private word R1 address to call after completing background floppy op R2 address to call after completing background winnie op R3 address to call to release FIQ after low level op

the layout of the descriptor block is offset length entry

     0      3 misc flags currently all 0
             bit 0 set if winnie needs FIQ
             bit 1 set if floppy needs FIQ
             bit 2 set if able to support background ops
     3      1 filing system number
     4      4 offset of filing system title from module base
     8      4 offset of boot text from module base
    12      4 offset of low level disc op entry from module base
    16      4 offset of miscellaneous low level from module base

&40542 Drives *

entry
R8 ->FileCore instance private word

exit
R0 default drive
R1 number of floppies
R2 number of winnies

&40543 FreeSpace *

entry
R0 ->zero terminated disc specifier R8 ->FileCore instance private word

exit
R0 if error V set r0-> error block else total free space on disc R1 largest object than can be created

Low level disc op call to parent module

Entry                                   Exit
R0 irrelevant                           IF successful 0 else ptr to error block
R1 reason code                          Unchanged
R2 disc address                         Incremented by amount transferred

R3 RAM address (or ptr to scatter list) Incremented appropriately

R4 length in bytes                      Decremented by amount transferred

R5 disc record describing disc
R6 boot block

                                        V set <=> error

R1 REASON CODE

bit 0 to bit 3

0       Verify                                          R2,R4
1       Read sectors                                    R2,R3,R4
2       Write sectors                                   R2,R3,R4

3       Read track on floppy, Read Id on hard discs     R2,R3
4       Write track                                     R2,R3

5       Seek                                            R2
6       Restore                                         R2

7       Step in, floppy only
8       Step out floppy only

15      Specify, hard discs only, DESCRIPTION BELOW     R2

Only reason codes 0,1,2,5,6 are essential. Seek is actually only used for park. Only reasn codes 0-2 modify R2-R4 on exit

bit 4

This bit is set if an alternate defect list for a hard disc is to be used. This is assumed to be in RAM 64 bytes after the start of the disc record R5

bit 5

If this bit is set rather than R3 being the RAM address for the transfer it is a pointer to a word aligned list of address,length word pairs for the transfer. All lengths except the final one must be a multiple of the sector size. These are used in turn until the total length as specified in R4 has been transferred. R3is incremented to point to the first pair that has not been fully used and this address,length pair is modified so that a subsequent call would continue from where this call has finished. This in only supported for reason codes 0 to 2.

bit 6

If this is set ESCAPE is ignored.

bit 7

If this bit is set the floppies will not timeout if the drive does not go ready within 1s but will wait indefinitely.

R2 disc address

bits 0 to 28
This specifies the start byte on the disc and must be at a sector or track boundary as appropriate. Sectors, heads and tracks are numbered from 0.

=
((track * heads + head) * sectors_per_track + sector - x) * sectors_size + byte

x is an adjustment for defect skipping applied for reason codes 0-2 with old map hard discs, and is the number of defective sectors earlier on the disc.

bits 29 to 31
This is the drive number, 0 to 3 floppies, 4 to 7 hard discs.

R5 Disc Record
entries marked * should be 0 for old map offset length

      0      1 log to base 2 of sector size
      1      1 sectors per track
      2      1 heads            (1 for L format floppy)
      3      1 density          1/2/4 single double quad if applicable

4 1 length of id field of map fragment in bits
5 1 log to base 2 of bytes for each map bit
6 1 track to track sector skew for random access file allocation 7 1 boot option
8 1 RESERVED
9 1 zones in the map
10 2 bits in zone which are neither map bits nor special zone 0 bytes

     12      4 disc address of root directory
     16      4 disc size in bytes

     20      2 disc id
     22     10 disc name

     32      4

R6 Boot Block

Sectors 12 and 13 of a hard disc contain a &200 byte boot block as below any unused bytes should be set to 0.

0                                                    1                 2
0                                                    C                 0
0                                                    0                 0

|                                        Hardware    | Disc record     |

|==>Defect list                          Specific <==| describing      |
|                                        Parameters  | disc            |

The drivers are responsible for doing defect skipping on old format hard discs. The defect list is a word list of disc addresses in bytes of the start of sectors which are defective. These do not have drive bits set and are absolute, rather than adjusted for earlier defects. The end of the defect list is marked by &200000xx where xx is a check byte calculated as follows

entry
Ra -> list start

exit

Ra      corrupt
Rb      check byte
Rc      corrupt

        MOV     Rb,#0           ;init check

loop

        LDR     Rc,[Ra],#4      ;get next entry
        CMPS    Rc,#&20000000   ;all done ?
        EORCC   Rb,Rc,Rb,ROR #13
        BCC     loop
        EOR     Rb,Rb,Rb,LSR #16;compress word to byte
        EOR     Rb,Rb,Rb,LSR #8
        AND     Rb,Rb,#&FF

&1BC-&1BF unadjusted parking disc address The boot block should also have the same check byte at the end as an old format map.

Low Level Miscellaneous

R0 reason code
R1 drive

0 Mount

entry
R2 disc address
R3 RAM address
R4 length to read
R5 disc rec to fill in

exit
IF error V set R0->error block else R0 preserved R1-R5 preserved

Floppy
This asks you to read in the free space map and identify the format. The suggested density to try first is given in the disc record. Identify the format consits of filling in the density and for old format sector size, sectors per track, heads, disc size and root dir.

Winnie
This asks you to read in the boot block if it doesn't have one you will have to fudge it

1 Poll changed

entry
R2 sequence number

exit
R2 sequence number
R3 result flags

The sequence number is to ensure no changes are lost due to reset being pressed. Both the parent module and the file core incarnation should start with a sequence number of 0 for each drive. The parent module increments the sequence number with each change of state. If the parent module finds the entry sequence number does not match its copy it should return changed/maybe changed depending on whether the disc changed line works/doesn't work.

result flags (exactly one of b0-b3 must be set) set<=>true b0 not changed
b1 maybe changed
b2 changed
b3 empty
b6 empty works may never return 0 after returning 1 for this drive b7 changed works may never return 0 after returning 1 for this drive