Deleting, Renaming, Writing at Random

Introduction

Three more things a program does to files: throw one away, give one a new name, and change part of one without rewriting the rest.

Delete: Function 19

The FCB names the file; function 19 marks each of its directory entries deleted by setting byte 0 to E5h - and nothing else. A is 0, or FFh if there was no such file. The data stays where it was until another file needs the blocks.

Rename: Function 23

Rename needs two names in one FCB: the old one in bytes 0-15 as usual, and the new one from byte 16 - a drive byte (0) and then the name and extension. The BDOS renames every entry of the file, so all its extents keep together. N builds the second name in FCB2 and copies its first 16 bytes up.

Write Random: Function 34

Function 34 is S9's read random the other way round: record number in bytes 33-35, the record at the DMA address, and only that record is written. A record past the end makes the file longer. Close afterwards, as always, so that the directory knows.

The Code

After the line JP Z,CMDW, add:

        CP 'E'
        JP Z,CMDE
        CP 'N'
        JP Z,CMDN
        CP 'P'
        JP Z,CMDP

After the last line of TR3 (RET), add:

; E name: erase a file.
CMDE:   CALL NEEDDOS
        LD DE,FCB
        CALL GETNAME
        JP C,WHAT
        LD DE,FCB
        LD C,19
        CALL 5
        CP 0FFH
        LD HL,NOTFM
        JP Z,DONE
        LD HL,ERASEM
        JP DONE

; N old new: rename. The new name goes in FCB bytes 16-31.
CMDN:   CALL NEEDDOS
        LD DE,FCB
        CALL GETNAME
        JP C,WHAT
        LD DE,FCB2
        CALL GETNAME
        JP C,WHAT
        LD HL,FCB2
        LD DE,FCB+16
        LD BC,16
        LDIR
        XOR A
        LD (FCB+16),A           ; the new name's drive byte: 0
        LD DE,FCB
        LD C,23
        CALL 5
        CP 0FFH
        LD HL,NOTFM
        JP Z,DONE
        LD HL,RENM
        JP DONE

; P name n text: write the text as record n, with write random.
CMDP:   CALL NEEDDOS
        CALL OPENIT
        CALL GETNUM
        JP C,WHAT
        LD (FCB+33),HL
        XOR A
        LD (FCB+35),A
        CALL TEXTREC
        LD DE,RECBUF
        LD C,26
        CALL 5
        LD DE,FCB
        LD C,34                 ; write random
        CALL 5
        LD DE,FCB
        LD C,16
        CALL 5
        LD HL,WROTEM
        JP DONE

After the line WROTEM: DEFB 'WRITTEN',13,10,0, add:

FCB2:   DEFS 36
ERASEM: DEFB 'ERASED',13,10,0
RENM:   DEFB 'RENAMED',13,10,0

Run it on a fresh work.mfi made from PRIMER.DSK (S10):

W 1:SCRAP.TXT GONE SOON
E 1:SCRAP.TXT
N 1:ROOMS.DAT PLACES.DAT
P 1:POEM.TXT 2 THE END.
T 1:POEM.TXT
Q

What you should see

> W 1:SCRAP.TXT GONE SOON
WRITTEN
> E 1:SCRAP.TXT
ERASED
> N 1:ROOMS.DAT PLACES.DAT
RENAMED
> P 1:POEM.TXT 2 THE END.
WRITTEN
> T 1:POEM.TXT
THE DISC
Forty tracks go round and round,
ten sectors on each one;
the directory shows what can be found,
and blocks say where they've gone.

Ask the DOS and it will look
the name up for you, line by line;
or read the sectors like a book
and all tTHE END.

3 RECORDS
>

Record 2 of POEM.TXT - the last third of the poem - is now THE END.; records 0 and 1 are untouched. In a fresh start of MAME with the same work.mfi:

0:DIR 1:
1: POEM    .TXT : PLACES  .DAT
1: DISKMON .COM
30k Size, 158k Free, 190k Total
0:

Change One Thing

E a file, then look at the directory sector with edisc.py dump after.dsk 2 0 (after mfi2dsk.py). Which byte of its entry changed?
P 1:POEM.TXT 5 MORE writes past the end. What record count will O 1:POEM.TXT show afterwards - and what is in records 3 and 4?

Summary

Function 19 deletes (status byte E5h), 23 renames every extent, 34 writes one record anywhere. With S10 that is everything a program needs to keep its own files.

S12: what DISKMON can - and cannot - do when the BDOS says no.

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