Jumping About: Any Record, at Once

Introduction

A game does not read its data file from the beginning every time the player walks into a room. It works out which record it wants and reads that one. The BDOS calls this random access.

Function 33

Put the record number - counting from 0 - in FCB bytes 33 and 34, low byte first, and 0 in byte 35. Set the DMA address, put the FCB in DE, 33 in C, and CALL 5. The record arrives at the DMA address, wherever it is in the file: the BDOS works out the extent and the block itself.

If there is no such record, nothing arrives and A is not 0. (S12 says what its values mean.)

The Code

R name n. GETNUM reads n into HL, which goes straight into bytes 33-34 with one LD (FCB+33),HL - the Z80 stores L first, which is the order the BDOS wants.

After the line JP Z,CMDT, add:

        CP 'R'
        JP Z,CMDR

After the last line of OPENIT (JP DONE), add:

; R name n: read record n with read random and show it.
CMDR:   CALL NEEDDOS
        CALL OPENIT
        CALL GETNUM
        JP C,WHAT
        LD (FCB+33),HL          ; the record number, low byte first
        XOR A
        LD (FCB+35),A
        LD DE,RECBUF
        LD C,26
        CALL 5
        LD DE,FCB
        LD C,33                 ; read random
        CALL 5
        OR A
        JR NZ,CMDR1
        LD HL,RECBUF
        LD BC,128
        CALL SHOWBLK
        JP MAIN

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

CMDR1:  PUSH AF
        LD HL,AM
        CALL PRINT              ; PRINT leaves A = 0, so keep it
        POP AF
        CALL PHEX
        JP CRMAIN

After the line RECSM: DEFB ' RECORDS',13,10,0, add:

AM:     DEFB 'A=',0

Run it on a copy of PRIMER.DSK:

R 1:ROOMS.DAT 150
R 1:ROOMS.DAT 200

What you should see

> R 1:ROOMS.DAT 150
0000 52 4F 4F 4D 20 31 35 30 ROOM 150
0008 3A 20 41 20 44 55 53 54 : A DUST
0010 59 20 48 41 4C 4C 2E 20 Y HALL.
0018 20 20 20 20 20 20 20 20
0020 20 20 20 20 20 20 20 20
0028 20 20 20 20 20 20 20 20
0030 20 20 20 20 20 20 20 20
0038 20 20 20 20 20 20 20 20
0040 20 20 20 20 20 20 20 20
0048 20 20 20 20 20 20 20 20
0050 20 20 20 20 20 20 20 20
0058 20 20 20 20 20 20 20 20
0060 20 20 20 20 20 20 20 20
0068 20 20 20 20 20 20 20 20
0070 20 20 20 20 20 20 20 20
0078 20 20 20 20 20 20 20 20
> R 1:ROOMS.DAT 200
A=01
>

Record 150 is in ROOMS.DAT's second extent, but you did not have to know that. ROOMS.DAT has 200 records, numbered 0-199, so there is no record 200.

Change One Thing

• Read records 0, 199 and 7 in that order. Does the order you ask in change anything?
• Take out XOR A / LD (FCB+35),A. GETNAME cleared the FCB, so it still works the first time. When might byte 35 not be 0?

Summary

Function 33 reads the record whose number is in FCB bytes 33-35, in any order, from anywhere in the file. That is all a program needs to keep its data on disc and fetch it as it goes.

Next

S10 writes a file.