FileCore Boot Block (changes)

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Boot Block

Overview and Format of Boot Block

On specific types of formatted discs, a boot block is used to store data that contains important information pertaining to the disc. Currently, FileCore’s boot block is 512 bytes long, and is stored at disc address &C00. The format of the boot block is as follows:

Note: In memory, this information would be stored in the order:

1. disc record
2. defect list
3. hardware parameters

This is to facilitate passing the values to FileCore SWIs.

Defect list

A defect list contains a list of known defective sectors on the disc. FileCore may use up to two lists to identify all defective sectors on the disc.

The method to determine whether a second defect list is present is by checking the big_flag byte in the disc record. If bit 0 is set, then the second defect list must be present.

Although both lists are similar in how they store the list of defective sectors, there are important differences that should be noted:

Example 1

Assuming only one list is required (due to an older, ‘small disc record’ being used), this example shows there are no defective sectors.

Example 2

Assuming two lists are required (due to the newer ‘large disc record’ being used, this example shows there are no defective sectors.

Calculating Defect List Checksum Byte

The checksum byte is only based on the previous entries contained in their respective list. i.e. checksum byte in list 2 does not compare against entries in list 1, and vice-versa.

The following Assembly code can be used to correctly calculate and update the checksum byte.

	MOV Rb,#0 ;init check
loop
	LDR Rc,[Ra],#4 ; get next entry
	CMPS Rc,#&20000000 ;all done ?
	EORCC Rb,Rc,Rb,ROR #13
	BCC loop
	EOR Rb,Rb,Rb,LSR #16 ;compress word to byte
	EOR Rb,Rb,Rb,LSR #8
	AND Rb,Rb,#&FF

Where on entry:
Ra = pointer to start defect list

Where on exit:
Ra = corrupt
Rb = checkbyte
Rc = corrupt

Hardware-dependent Information

As the name suggests, this part of the boot block is used to store information that is specific to the underlying hardware, and as a result will vary in size and contents.

The Boot Block’s Disc Record

The purpose of the boot block disc record is to give the necessary information to find the disc’s map. You should not rely on the information it contains for any other purpose, unless it is unavailable in the disc’s map. Consequently:

• For an old map disc, you should use the boot block’s disc record to find the map. If information you require is held in the map, you must use that in preference to the boot block’s disc record
• For a new map disc, you should use the boot block’s disc record to find the map. Once you have found the map you should then always use its disc record, rather than the boot block’s

More information on the disc record is found here.

The non-ADFS Partition Descriptor

The purpose of this part of the boot block is to decribe any non-ADFS partition on the disc. Such a partition must come at the end of the disc, and is excluded from all descriptions of the ADFS partition.

You can calculate the disc address of the start of the non-ADFS partition as follows:

start cylinder × heads on drive × sectors per track × bytes per sector

Boot Block Checksum Byte

The last byte of the boot block is a checksum. The value of the byte is calculated as follows:

Perform an 8 bit add with carry on each of the other bytes in the block, starting with value 0

CheckSum ROUT
	STMFD R13!, {R1, LR}
	ADDS LR, R0, R1 ;->end+1 C=0
	SUB R1, LR, #1 ;->check byte
	MOV R2, #0
	B %FT20
10
	LDRB LR, [R1,#-1] ! ;get next byte
	ADC R2, R2, LR ;add into checksum
	MOVS R2, R2, LSL #24 ;bit 8 = carry
	MOV R2, R2, LSR #24
20
	TEQS R0, R1
	BNE %BT10 ;loop until done
	LDMFD R13!, {R1, LR}

Where on entry:
R0 = Start
R1 = block length

Where on exit:
R0 = Preserved
R1 = Preserved
R2 = Checksum

Please note that the checksum does not include the last byte (itself).

See also

• FileCore Overview
• FileCore Technical Details