The Directory, by Hand

Introduction

DIR is the DOS reading the directory for you. DISKMON can now read any sector - so it can read the directory, and list the files, itself.

Four Sectors, Sixty-Four Entries

READDIR reads track 2, sectors 0 to 3, one after another into DIRBUF, 512 bytes apart: 2,048 bytes, 64 entries. It moves the address on by adding 2 to H - 512 is 200h - and keeps its counter and address on the stack across each call, since call 162 changes BC and HL.

CMDD walks the 64 entries with IX, 32 bytes at a time. An entry whose byte 0 is E5h is empty or deleted; every other one is shown by SHOWENT: the name and extension (each byte without its top bit, since bit 7 of byte 9 is the locked flag), a * if locked, the extent (byte 12) and the record count (byte 15).

PDEC prints a number in decimal, using DIV10 - the Z80 has no divide instruction, so DIV10 does long division by 10 a bit at a time.

The Code

After the line JP Z,CMDNXT, add:

        CP 'D'
        JP Z,CMDD

After the last line of DISCERR (JP CRMAIN), add:

; D d: list the files on drive d by reading the directory itself.
CMDD:   CALL GETNUM
        JP C,WHAT
        LD (SDRV),A
        CALL READDIR
        JP NZ,DISCERR
        LD IX,DIRBUF
        LD B,64
CMDD1:  PUSH BC
        LD A,(IX+0)
        CP 0E5H                 ; E5h: an empty or deleted slot
        JR Z,CMDD2
        CALL SHOWENT
CMDD2:  LD DE,32
        ADD IX,DE
        POP BC
        DJNZ CMDD1
        JP MAIN

; READDIR: track 2 sectors 0-3 of drive SDRV into DIRBUF. Z if all read.
READDIR: LD A,2
        LD (STRK),A
        LD HL,DIRBUF
        LD B,0
RD1:    PUSH BC
        PUSH HL
        LD A,B
        LD (SSEC),A
        CALL SETPAD
        POP HL
        LD (0FB53H),HL          ; this sector's place in DIRBUF
        PUSH HL
        RST 8
        DEFB 162
        POP HL
        POP BC
        RET NZ
        INC H                   ; 512 bytes on: H + 2
        INC H
        INC B
        LD A,B
        CP 4
        JR NZ,RD1
        XOR A                   ; Z: all four read
        RET

; SHOWENT: one directory entry at IX: name, extent, record count.
SHOWENT: PUSH IX
        POP HL
        INC HL
        LD B,8
        CALL PNAME
        LD A,'.'
        CALL PUTC
        LD B,3
        CALL PNAME
        BIT 7,(IX+9)
        LD A,' '
        JR Z,SE1
        LD A,'*'                ; bit 7 of byte 9: locked
SE1:    CALL PUTC
        LD HL,EXTM
        CALL PRINT
        LD L,(IX+12)
        LD H,0
        CALL PDEC
        LD HL,RECM
        CALL PRINT
        LD L,(IX+15)
        LD H,0
        CALL PDEC
        JP NEWLINE

; PNAME: print B name characters from HL, without their top bits.
PNAME:  LD A,(HL)
        AND 7FH
        CALL PUTC
        INC HL
        DJNZ PNAME
        RET

After the line BADM: DEFB ': BAD DATA',13,10,0, add:

EXTM:   DEFB ' EXTENT ',0
RECM:   DEFB ' RECORDS ',0

Run it with a copy of FILES.DSK, from Inside the Einstein: The Disc, in drive 1:

D 1

What you should see

> D 1
README  .TXT* EXTENT 0 RECORDS 1
STORY   .TXT  EXTENT 0 RECORDS 128
STORY   .TXT  EXTENT 1 RECORDS 29
NUMBERS .BIN  EXTENT 0 RECORDS 24
DISKMON .COM  EXTENT 0 RECORDS 13
>

One line per entry, not per file: STORY.TXT is over 16K, so it has two. And the * on README.TXT is bit 7 of its byte 9.

Change One Thing

• Remove the CP 0E5H test. What will D 1 print for the empty slots?
• Make DISKMON print each file once, not once per extent. (Hint: only entries whose extent is 0.)

Summary

DISKMON reads the directory with four MOS calls and decodes the entries itself: the DOS's DIR, done by hand.

Next

S17 follows an entry's blocks to the file's data.