Reading a File, Record by Record

Introduction

The file is open. Now DISKMON reads it - the way Blackwater Hall reads its text, and the way every program under the DOS reads a file: a record of 128 bytes at a time.

Where the Bytes Go: the DMA Address

A read does not hand you the bytes in registers. It puts all 128 of them in memory, at the DMA address. When a program starts, the DMA address is 0080h; function 26, with an address in DE, moves it. DISKMON points it at its own RECBUF, at 8A00h, before it reads.

Function 20: the Next Record

Function 20, with the FCB in DE, reads the next record of the file into the DMA address and moves on. A comes back 0 each time a record was read, and 1 when there are no more - nothing is written to the buffer then. You do not have to think about extents: when one ends, the BDOS finds the next by itself.

A text file's last record is filled out with 1Ah bytes after the text, so TYPEOUT stops printing at the first 1Ah.

The Code

T name: open the file (OPENIT gives up with NOT FOUND if it cannot), set the DMA address, then read and print until A is not 0 or the text ends, and count the records.

After the line JP Z,CMDO, add:

        CP 'T'
        JP Z,CMDT

After the last line of CMDO (JP MAIN), add:

; T name: type a text file, a record at a time, through the BDOS.
CMDT:   CALL NEEDDOS
        CALL OPENIT
        LD DE,RECBUF            ; reads go to RECBUF
        LD C,26
        CALL 5
        LD HL,0
        LD (COUNT),HL
CMDT1:  LD DE,FCB
        LD C,20                 ; read the next record
        CALL 5
        OR A
        JR NZ,CMDT2             ; not 0: the end of the file
        LD HL,(COUNT)
        INC HL
        LD (COUNT),HL
        LD HL,RECBUF
        LD BC,128
        CALL TYPEOUT
        JR NC,CMDT1             ; carry: a 1Ah ended the text
CMDT2:  CALL NEWLINE
        LD HL,(COUNT)
        CALL PDEC
        LD HL,RECSM
        JP DONE

; OPENIT: name from the line into FCB, open it; to MAIN if that fails.
OPENIT: LD DE,FCB
        CALL GETNAME
        JP C,WHAT
        LD DE,FCB
        LD C,15
        CALL 5
        CP 0FFH
        RET NZ
        LD HL,NOTFM
        JP DONE

After the last line of PDEC (JP PUTC), add:

; TYPEOUT: print BC bytes from HL as text, stopping at 1Ah (carry set).
TYPEOUT: LD A,(HL)
        CP 1AH
        SCF
        RET Z
        CALL PUTC
        INC HL
        DEC BC
        LD A,B
        OR C
        JR NZ,TYPEOUT
        RET                     ; carry clear: no 1Ah yet

After the line NOTFM: DEFB 'NOT FOUND',13,10,0, add:

COUNT:  DEFW 0
RECSM:  DEFB ' RECORDS',13,10,0

Run it on a copy of PRIMER.DSK:

T 1:POEM.TXT

What you should see

> 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 the work is yours and mine.

3 RECORDS
>

Change One Thing

Take out the function 26 call in CMDT. Where will the records go now, and what will T print?
POEM.TXT is three records. If TYPEOUT did not stop at 1Ah, what would you see after the last line?

Summary

Function 26 says where records go; function 20 fetches the next one, with A = 0 while there are more and 1 at the end. DISKMON can show any text file on a disc.

S9 stops reading in order and jumps straight to any record.

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