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Controller PCB
3-22
The 8-bit data register (which actually consists of several registers in a stack with
only one register presented to the data bus at a time) stores data, commands,
parameters, and FDD status information. Data bytes are written into the data
register to program the FDC and are read out of it to obtain results after a
command.
The FDC can perform 15 different commands. Each command is initiated by a
multibyte transfer from the processor, and the result after execution of the com-
mand can also be a multibyte transfer back to the processor. Because of this
multi byte interchange of information between the FDC and the processor, each
command can be considered in three phases.
• Command Phase - The FDC receives all information required to perform a
particular operation from the processor.
• Execution Phase - The FDC performs the operation.
• Result Phase -
Status and other housekeeping information are made
available to the processor.
I
CLOCK &
I------<~I
WRITE
I . . . . .
TIMING
-.J
PRECOMP
11---1><>-----'
CIRCUIT
I
-I
CIRCUIT
....
~
I
~:;:E
VFO SYNC ...
DATA WINDOW
A
RD DATA
":::!
FLEXIBLE
H
DECODER
L
..hl'RIVE B SELECT
DISK
H
DR IV\<: A SELECT
DMA
A
LSI
..
DIRECTION
CONTROLLER
...
STEP
,
r
i
REQUEST.....
....
DMA
-C>o-"----I
...
WRITE GATE
ACKNOWL~DGE
...
HEAD LOAD
INTERRUPT
REQUEST
Figure 3-11 FDe Block Diagram
. .
SIDE SELECT
...
MFM
LOW CURRENT
..
FILE UNSAFE RESET
A
..
READY
... WRITE PROTECT
A
......,-
TWO SIDED
A
TRACKO
...
.,.. INDEX
A
FILE UNSAFE
fDD
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