Tandy 1000 HX Technical Reference Manual page 218

NEC
M
P D 7 6 5 A / L J > D 7 2 6 5
tively. Also, the loss of a ready signal at the beginning of
a command execution phase causes bits 7 and 6 of sta-
tus register 0 to be set to 0 and 1, respectively.
Table 7 shows the relationship between N, SC, and GPL
for various sector sizes.
Table 7. Sector Size
3V2" Sony Micro Floppydisk
FMMode 128 Bytes / Sector
256
512
MFM Mode(4) 256
512
1024
0
1
2
1
2
3
OF
09
05
OF
09
05
07
OE
1B
OE
1B
35
1B
2A
3A
36
54
74
Note:
(1) Suggested values of GPL in Read or Write commands to avoid
splice point between data field and ID field of contiguous sections.
(2) Suggested values of GPL in format command.
(3) All values except sector size are hexidecimal.
(4) In MFM mode FDC cannot perform a Read/Write/Format operation
with 128 bytes/sector. (N = 00).
Scan Commands
The Scan commands allow data which is being read
from the diskette to be compared against data which is
being supplied from the main system. The FDC com-
pares the data on a byte-by-byte basis and looks for a
sector of data which meets the conditions of
DFDD = Dprocessor. DRDD < Dp
rO
cessor. or D F D D >
D
Processor- The hexidecimal byte of FF either from
memory or from FDD can be used as a mask byte be-
cause it always meets the condition of the comparison.
One's complement arithmetic is used for comparison
(FF = largest number, 00 = smallest number). After a
whole sector of data is compared, if the conditions are
not met, the sector number is incremented (R + STP —
R), and the scan operation is continued. The scan opera-
tion continues until one of the following conditions oc-
cur: the conditions for scan are met (equal, low, or high),
the last sector on the track is reached (EOT), or the termi-
nal count signal is received.
If the conditions for scan are met, then the FDC sets the
SH (scan hit) flag of status register 2 to a 1 (high) and ter-
minates the Scan command. If the conditions for scan
are not met between the starting sector (as specified by
R) and the last sector on the cylinder (EOT), then the
FDC sets the SN (scan not satisfied) flag of status regis-
ter 2 to a 1 (high) and terminates the Scan command. The
receipt of a terminal count signal from the processor or
DMA controller during the scan operation will cause the
FDC to complete the comparison of the particular byte
which is in process and then to terminate the command.
Table 8 shows the status of bits SH and SN under vari-
ous conditions of Scan.
Table 8. Scan Conditions
Command
Scan Equal
Scan Low or
Equal
Scan High or
Equal
Status Register 2
Bit2 = SN
0
1
0
0
1
0
0
1
Bit3 = SH
1
0
1
0
0
1
0
0
Comments
DFDD =
D
Processor
DFOD^DProcessor
DFDD
=
Dprocessor
DFDD
<D
Processor
DFDD
>D
Processor
DFDD =
D
Processor
DFDO
>D
Processor
DFDD
<D
Processor
If the FDC encounters a deleted data address mark on
one of the sectors (and SK = 0), then it regards the sec-
tor as the last sector on the cylinder, sets the CM (con-
trol mark) flag of status register 2 to a 1 (high) and
terminates the command. If SK = 1, the FDC skips the
sector with the deleted address mark and reads the next
sector. In the second case (SK = 1), the FDC sets the CM
Format
8" Standard Floppy
FM Mode
MFM Mode(4)
5 W ' Minifloppy
FM Mode
MFM Mode(4)
Sector S i n
128 Bytes/Sector
256
512
1024
2048
4096
256
512
1024
2048
4096
8192
128 Bytes/Sector
128
256
512
1024
2048
256
256
512
1024
2048
4096
N
00
01
02
03
04
05
01
02
03
04
05
06
00
00
01
02
03
04
01
01
02
03
04
05
sc
1A
OF
08
04
02
01
1A
OF
08
04
02
01
12
10
08
04
02
01
12
10
08
04
02
01
OM.(1)
07
OE
1B
47
C8
C8
OE
1B
35
99
C8
C8
07
10
18
46
C8
C8
OA
20
2A
80
C8
C8
OPL(2,3)
1B
2A
3A
8A
FF
FF
36
54
74
FF
FF
FF
09
19
30
87
FF
FF
OC
32
50
FO
FF
FF