Commodore Computer Commodore 64 Uživatelský manuál stáhnout pdf (Strana 32)

BLOCK.ALLOCAIE

In order to safely use random files along with regular files,your programs

must check the BAM to find available blocks, and change the BAM to reflect

that you've used them. Once you update the BAM,your random tiles will be

safe-at least unless you perform the VALIDATE command (see chapter 3).

FORMAT FOR THE BLOCK-ALLOCATE COMMAND:

PRINT# file#, "BLOCK.ALLOCATE:" drive, track, block

How do you know which blocks are available to use? If you try a block

that isn't available, the DOS will set the error message to number 65, NO

BLOCK, and set the track and block numbers to the next available track and

block number. Therefore, any time you attempt to write a block to the disk,

you must first try to allocate that block. If that block isn't available, read the

next block available from the error channel and then allocate that block.

EXAMPLE OF PROCEDURE TO ALLOCATE BLOCK:

10OPEN15.8,15

20 OPEN5,8,5, "# DRIVE

30PRINT#5,"DATA" / ~

40T=I:S=1 ~ ~

50 PRINT#15,"B-A:"0,T, S~

60 INPUT#15, A, B$, C, D

70 IF A=65 THEN T=C: S=D: GOTO 50

80 PRINT# 15, "B.W:" 5,0, T, S

BLOCK-FREE

The BLOCK.FREE command is the opposite of BLOCK.ALLOCATE, in

that it frees a block that you don't want to use anymore for use by the system.

It is vaguely similar to the SCRATCH command for files, sinceit doesn't really

erase any data from the disk-just frees the entry, in this casejust in the BAM.

FORMAT FOR BLOCK.FREE COMMAND:

PRINT# f1le#, "BLOCK-FREE:" drive, track, block

or abbreviated as

PRINT# file#, "B.F:" drive, track, block

UsingRandom Files

The only problem with what you've learned about random fIles so far is

1 2 ... 27 28 29 30 31 32 33 34 35 36 37 ... 64 65

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Commodore Computer Commodore 64 Uživatelský manuál Strana 32