The raw MFM data that must be presented to the disk controller will be twice as large as
the unencoded data. The following Table shows the relationship:
1 ---> 01
0 ---> 10 ;if following a 0
0 ---> 00 ;if following a 1
With clever manipulation, the blitter can be used to encode and decode the MFM.
In one common form of GCR recording, each data byte always has the most significant bit
set to a 1. MSBSYNC, when a 1, tells the disk controller to look for this sync bit on every
disk byte. When reading a GCR formatted disk, the software must use a translate Table
called a nybbleizer to assure that data written to the disk does not have too many
consecutive 1's or 0's.
DSKSYNC - DISK INPUT SYNCHRONIZER
The DSKSYNC register is used to synchronize the input stream. This is highly useful when
reading disks. If the WORDSYNC bit is enabled in ADKCON, no data is transferred until a
word is found in the input stream that matches the word in the DSKSYNC register. On
read, DMA will start with the following word from the disk. During disk read DMA, the
controller will resync every time the word match is found. Typically the DSKSYNC will be
set to the magic MFM sync mark value, $4489.
In addition, the DSKSYNC bit in INTREQ is set when the input stream matches the
DSKSYNC register. The DSKSYNC bit in INTREQ is independent of the WORDSYNC enable.
DISK INTERRUPTS
The disk controller can issue three kinds of interrupts:
o DSKSYNC (level 5, INTREQ bit 12) - input stream matches the DSKSYNC register.
o DSKBLK (level 1, INTREQ bit l) - disk DMA has completed.
o INDEX (level 6, 8520 Flag pin) - index sensor triggered.
Interrupts are explained further in the section "Length, Direction, DMA Enable". See
Chapter 7, "System Control Hardware," for more information about interrupts. See
Appendix F for more information on the 8520.
- 244 Interface Hardware -
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