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The C Users' Group Library 1994 August
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v_06_07
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v6n7018c.txt
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1989-09-28
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Table 3 -- Command and Result Sequences for floppy commands
-----------------------------------------------------------
Byte r/w Read ID Format Recal Seek
C-0 cmd+MT cmd+MFM cmd+MFM cmd cmd
+MFM+SK
C-1 HDS+DS HDS+DS HDS+DS DS HDS+DS
C-2 C N NCN
C-3 H SC
C-4 R GPL
C-5 N D
C-6 EOT
C-7 GPL
C-8 DTL
R-0 ST0 ST0 ST0
R-1 ST1 ST1 ST1
R-2 ST2 ST2 ST2
R-3 C C C **
R-4 H H H **
R-5 R R R **
R-6 N N N **
** For format, these have no meaning.
Table 4 -- Command and Result Sequences for FDC commands
--------------------------------------------------------
Byte Specify Sense Intr Status Sense Drive Status
C-0 cmd cmd cmd
C-1 SRT+HUT HDS+DS
C-2 HLT+ND
R-0 ST0
R-1 PCN ST3
Table 5 -- What the symbols mean
--------------------------------
cmd - bit 0-4, command code: see table 6. Bit 4 only set for "scan"
commands, which are not discussed here.
SK - bit 5, Skip deleted address mark. This article does not cover
deleted data.
MFM - bit 6, Double Density (Modified Frequency Modulated).
MT - bit 7, Multi-track. Really should be 'multi-head'. Allows a read
data or read deleted data to continue at the end of track onto the
other side of floppy. Do not use for writing.
DS - bits 0 & 1, Drive select
HDS - bit 2, Head select.
C - Cylinder. A part of the sector ID, corresponding to (but not
necessarily equal to) the position of the track.
H - Head. A part of the sector ID, corresponding to the (but not
necessarily equal to) the side of the disk.
R - Record. A part of the sector ID, generally called the sector number.
Definitely NOT the same as a CPM record. May or may not be ordered
sequentially.
N - Number of bytes code. Defines sector size: see table 7 for value.
EOT - End of Track. The last possible sector number on the track. Can
be larger than the number of physical sectors.
GPL - Gap Length. Used by FDC when reading or writing.
FGPL - Format Gap Length. Used by FDC in formatting. It is a different
value than the GPL used in read/write. See Table 7 for value.
DTL - Data Length. Number of bytes in sector if N == 0.
SC - Sectors per cluster. Format uses to know how much to write
D - Data Fill byte for format command. Initial data written to sectors.
NCN - New Cylinder Number. This is the position to "SEEK" to.
HUT - bits 0-3, Head Unload time. IBM XT specs 0xF (240 msec).
SRT - bits 4-7, Step Rate time. (16-SRT) = # millisecond. IBM XT specs
0xC, which XT Tech Ref says is 6 ms track-to-track (I figure it to
be 4 ms).
ND - bit 0, Non-DMA mode. I don't recommend this: should remain clear.
HLT - bits 1-7, Head Load Time. 2 ms increments. XT Tech Ref specs
0x1.
ST0-ST3 - Status registers ST0, ST1, ST2, and ST3. See table 8 for
detail.
PCN - Present Cylinder Number. Where the FDC thinks the head is after a
seek or recalibrate.
Table 6 -- floppy disk command codes (except scan commands)
-----------------------------------------------------------
0x00 Invalid
0x01 Invalid
0x02 Read Track (all sectors)
0x03 Specify drive parameters
0x04 Sense drive status (get Status Reg 3)
0x05 Write Data (1 or more sectors)
0x06 Read Data (1 or more sectors)
0x07 Recalibrate drive (move head to cylinder 0)
0x08 Sense interrupt status (MUST be issued immediately after Seek
or Recalibrate)
0x09 Write deleted data - not recommended
0x0A Read ID fields (read ID fields off disk, wherever the head
currently is).
0x0B Invalid
0x0C Read deleted data - not recommended
0x0D Format a track (the lowest level format)
0x0E Invalid
0X0F Seek (move head to desired cylinder)
Table 7 -- Values for N, GPL, and EOT for 5 1/4" floppies
---------------------------------------------------------
Density size N SC GPL FGPL
single 128 0x00 0x12 0x07 0x09
single 128 0x00 0x10 0x10 0x19
single 256 0x01 0x08 0x18 0x30
single 512 0x02 0x04 0x46 0x87
single 1024 0x03 0x02 0xC8 0xFF
single 2048 0x04 0x01 0xC8 0xFF
double 256 0x01 0x12 0x0A 0x0C
double 256 0x01 0x10 0x20 0x32
double 512 0x02 0x08 0x2A 0x50
double 1024 0x03 0x04 0x80 0xF0
double 2048 0x04 0x02 0xC8 0xFF
double 4096 0x05 0x01 0xC8 0xFF
Note: In the IBM PC, the format is double, 512 bytes. Despite the
information in this table, nine sectors are put on the disk
successfully.
Table 8 -- Status Register bit definitions
------------------------------------------
Reg bit sym Description
ST0 6&7 IC Interrupt Code: 00= normal, 01= Abnormal termination,
10= Invalid command, 11= Aborted: ready signal changed.
5 SE Seek End.
4 EC Equiptment check. Fault signal or could not recalibrate
3 NR Not Ready. Normally because door not closed.
2 HD Head Address. 0 or 1.
0&1 US Unit (drive) select. Only needed when doing overlapping
seeks.
ST1 7 EN End of cylinder. An attempt to read past the last sector
on a track.
6 n/a (always zero)
5 DE Data Error. Bad data read from floppy.
4 OR Over run. The processor did not get data fast enough.
3 n/a (always zero)
2 ND No Data. Could not read ID field requested.
1 NW Not Writable. Write protected floppy.
0 MA Missing Address Mark. Could not find ID field requested.
(This often occurs with DE with a unformatted or
corrupted disk.
ST2 7 n/a (always zero)
6 CM Control Mark. A deleted sector was detected.
5 DD Data error in Data field. Bad data on floppy.
4 WC Wrong Cylinder. "C" field in command doesn't match the
corresponding field in the ID field on disk.
3 SH Scan Equal Hit.
2 SN Scan not Satisfied.
1 BC Bad Cylinder. same as WC, but "C" == 0xFF.
0 MD Missing Data address mark. Could not find Data address
mark field.
ST3 7 FT Fault. Signal from floppy disk drive (FDD).
6 WP Write Protected. Signal from FDD.
5 RDY Ready. Signal from FDD.
4 T0 Track 0. Signal from FDD.
3 TS Two Side. Signal from FDD.
2 HD Head address. Signal from FDD.
0&1 US Unit select. Signals from FDD.
Note: The "sym" column is the mnemonic assigned in the Intel
documentation.