Tandy 1000 Technical Reference Manual

Tandy 1000
Technical Reference Manual
8272A
Table 9. Sector Size Relationships.
FORMAT SECTOR SIZE N SC GPl
1
GPL
2
REMARKS
SECTOR SIZE N
SC
GPL' GPl
2
FM Mode 128 bylesiSeelar
00
lA 07 lB IBM Diskette 1
128
byles/Sector 00
12
07
09
256
01
OF
OE 2A IBM Diskette 2
128 00 10 10 19
512 02 08
lB
3A
256
01
08
18
30
1024
03
O'
47 8A
512
02
O. .6
B7
2048
O.
02
C8
FF
1024
03 02 C8
FF
'096
05
01
C8
FF
2048
O'
01
C8
FF
MPM Mode 256 01
lA
OE
36
IBM D,skelte 20
256
01
'2
OA
OC
512 02
OF
'B 5'
256
0' '0
20 J2
1024
03 08 35
7.
IBM DisketTe 20 512
02 08
2A
50
2048
O'
O'
gg
FF
1024
03
O'
BO
FO
'096
D5 02 C~
FF
2048
O'
02
C8
FF
8192 06
01
C8
FF
4096
05
0'
C8
FF
Nole: 1 SUQgested values of GPl in Read or Write Commands to avoid splice pOint between dala tleld and 10 field 01 contlQuOus sections
2 Suggested values or GPL In format command
SCAN COMMANDS
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 CM (Control
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 (Processor in
NON-DMA mode, and DMA Controller in DMA mode)
The FDC
compares the data on a byte-by-byte basis, and
looks for a sector of data which meets the conditions of
D
FDD
=DProcessor, D
FDD
~ DProcessor, or D
FDD
~ DProcessor
Ones 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 (EaT), or the ter·
minal
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
terminates 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 (EaT).
then
the FDC sets the SN (Scan Not Satisfied) flag of
Status Register 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 com·
parison of the particular byte which is in process, and
then
to terminate the command. Table 10 shows the
status of bits SH and SN under various conditions of
SCAN
When either the STP (contiguous sectors STP = 01, or
alternate sectors STP =02 sectors are read) or the MT
(Multi-Track) are
programmed, it is necessary to
remember that the last sector on the track must be read
For
example, if STP =02, MT = 0, the sectors are
numbered sequentially 1 through 26, and we start the
Scan Command at sector 21; the following will happen.
Sectors 21, 23, and 25 will be read, then the next sector
(26) will be skipped and the Index Hole will be en·
countered before the EaT value of 26 can be mad This
will result in an abnormal termination of the command.
If the EaT had been set at 25 or the scanning started at
sector 20, then the Scan Command would be completed
in a normal manner
The read/write head within the FDD IS moved from
cylinder to cylinder under control of the Seek Command
The FDC compares the PCN (Present Cylinder Number)
which is the current head position with the NCN (New
Cylinder Number), and performs the following operation
if there is a difference:
PCN < NCN: Direction signal to FDD set to a I (high),
and
Step Pulses are issued. (Step In)
PCN > NCN: Direction signal to FDD set to a a(low)
and
Step Pulses are issued (Step OuL)
The rate at
which Step Pulses are issued is controlled by
SRT
(Stepping Rate Time) in Ihe SPECIFY Comrnalld
After each Step Pulse is issued NCN IS cornpared
against PCN, and when NCN = PCN, then the SE (Seek
End) flag is
set in Status Register ato a 1 (high). and the
command IS terminated
During the Scan Command data is supplied bv either the
processor or DMA Controller for comparison against the
data read from the diskette. In order to avoid nav'og the
OR (Over Run) flag set In Status Register 1, It is nec-
essarv to have the data available In less than 27 ~s IFM
Mode) or 13 ~s (MFM Mode) If an Overrun occurs tne
FDC terminates the command
SEEK
Mark) flag of Status Register 2 to a 1 (high) and ter·
minates the command. If SK = 1, the FDC skips the sec-
tor with the Deleted Address Mark, and reads the next
sector. In the second case (SK = 1), the FDC sets the CM
(Control Mark) flag of Status Register 2 to a 1 (high) in
order to show that a Deleted Sector had been en·
countered.
D
FDO
= Dprocessor
DFDD* Dprocesso'
Table 10. Scan Status Codes
Scan Equal
Scan Low Ot Equal
Scan HIQhor Equal
6-233

Tandy 1000 Specifications

General

Processor	Intel 8088
Processor Speed	4.77 MHz
RAM	128 KB (expandable to 640 KB)
Operating System	MS-DOS 2.11
Manufacturer	Tandy Corporation
Release Year	1984
Storage	Single or double 5.25" floppy disk drive (360 KB), optional hard drive
Sound	Tandy 3-voice sound
Ports	Parallel, Serial
Graphics	CGA (Color Graphics Adapter)
Graphics Modes	320x200, 640x200