Tandy 1000 SL Technical Reference Manual page 195

Data command. (Status register 0 also has bits 7 and 6
set to 0 and 1, respectively.)
The Write command operates in much the same man-
ner as the Read command. The following items are the
same, and one should refer to the Read Data command
for details:
Transfer capacity
EN (end of cylinder) flag
ND (no data) flag
Head unload time interval
ID Information when the processor terminates
command
Definition of DTLwhen N = O and when N#O
In the write data mode, data transfers between the proc-
essor and FDC, via the data bus, must occur every 27ps
in the FM mode and every 13ps in the MFM mode. If the
time interval between data transfers is longer than this,
the FDC sets the OR (overrun) flag in status register 1
to a 1 (high) and terminates the Write Data command.
(Status register 0 also has bits 7 and 6 set to 0 and
1 ,
respectively.)
Write Deleted Data
This command is the same as the Write Data command
except a deleted data address mark is written at the be-
ginning of the data field instead of the normal data ad-
dress mark.
Read Deleted Data
This command is the same as the Read Data command
except that when the FDC detects a data address mark
at the beginning of a data field (and SK= 0 (low)), it will
read all the data in the sector and set the CM flag in sta-
tus register 2 to a 1 (high), and then terminate the com-
mand. If SK
=
1, then the FDC skips the sector with the
data address mark and reads the next sector.
Read a Track
This command is similar to the Read Data command ex-
cept that this is a continuous read operation where the
entire data field from each of the sectors is read. Imme-
dlatelyafter sensing the index hole, the FDC starts read-
ing all data fields on the track as continuous blocks of
data. If the FDC finds an error in the ID or data CRC
check bytes, it continues to read data from the track.
The FDC compares the ID information read from each
sector with the value stored in the IDR and sets the ND
flag of status registerl toal(high)if thereisnocompari-
son. Multi-track or skip operations are not allowed with
this command.
This command terminates when the number of sectors
read is equal to E a . If the FDC does not find an ID ad-
dress mark on the diskette after it senses the index hole
for the second time, it sets the MA (missing address
mark) flag in status registerl t o a l (high) and terminates
the command. (Status register 0 has bits 7 and 6 set to 0
and 1, respectively.)
Read ID
The Read ID command is used to give the present posi-
tion of the recording head. The FDC stores the values
from the first ID fteld it is able to read. If no proper ID
address mark is found on the diskette before the index
hole is encountered for the second time, then the MA
(missing address mark) flag in status register 1 is set to a
1 (high), and if no data is found then the ND (No data)
flag is also set in status register 1 to a 1 (high). The com-
mand is then terminated with bits7and 6 in status regis-
ter 0 set to 0 and 1, respectively. During this command
there is no data transfer between FDC and the CPU ex-
cept during the result phase.
Format a Track
The Format a Track command allows an entire track to
be formatted. After the index hole is detected, data is
written on the diskette; gaps, address marks, ID fields,
and data fields, all per the IBM System 34 (double den-
sity) or System 3740'(single density) format, are re-
corded. The particular format which will be written is
controlled by the values programmed into N (number of
byteslsector), SC (sectorslcylinder), GPL (gap length),
and D (data pattern) which are supplied by the proces-
sor during the command phase. The data field is filled
with the byte of data stored in D. The ID field for each
sector is supplied by the processor; that is, four data re-
quests per sector are made by the FDC for C (cylinder
number), H (head number), R (sector number), and N
(number of byteslsector). This allows the diskette to be
formatted with nonsequential sector numbers, i f
desired.
The processor must send new values for C, H, R, and N
to the pPD765AlpPD7265 for each sector on the track. If
FDC is set for the DMA mode, it will issue four DMA re-
quests persector. If it isset forthe interrupt mode, it will
issue four interrupts per sector and the processor must
supply C, H, R, and N loads for each sector. The con-
tents of the R register are incremented by 1 after each
sector is formatted; thus, the R register contains a value
of R when it is read during the result phase. This incre-
menting and formatting continues for the whole track
until the FDC detects the index hole for the second
time, whereupon it terminates the command.
If a fault signal is received from the FDD at the end of a
write operation, then the FDC sets the EC flag of status
register0 to a 1 (high) and terminates the command after
setting bits7and6of statusregisterOtoOand1, respec-
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