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CHAPTER 1
FLOPPY DISKETTES (RXA:)
Diskettes are block-addressable devices, each of which
contains 77 tracks of 26 sectors each. No file structure is
defined by the TOPS-10 monitor, although it is possible for
a user-mode program to define a file structure.
Diskettes may be recorded in either single or double
density. Single and double density may not be mixed on a
single diskette. Single density diskettes record 64 16-bit
words per sector. Double density diskettes record 128
16-bit words per sector. The diskette's density is
determined by the monitor during each IN, INPUT, OUT, and
OUTPUT UUO, and the file status (IO.DEN) is updated
accordingly. This allows your program to determine the
density of a diskette by simply reading a block and
examining the resultant file status.
Only 32 bits of each 36-bit data word in your program's
buffer are read or written. Bits 0, 1, 18, and 19 are not
transferred and are indeterminate. Bits 2 through 17 are
transferred first.
The monitor provides two basic addressing modes for the
diskette. Your program specifies which addressing mode is
used for each IO operation in the file status word.
In logical addressing mode, several physical sectors are
grouped to form a logical block, which is transmitted with a
single IN, INPUT, OUT, or OUTPUT UUO. Logical blocks are
numbered sequentially from zero. Physical track zero is not
accessible in logical addressing mode because it is reserved
for potential ANSI standardization. The physical sectors
comprising each logical block are not contiguous; they are
assigned in such a manner as to optimize hardware
performance. The monitor provides two logical addressing
modes; one (Logical-11) is compatible with Digital's PDP-11
17-September-1984 15:17:21
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FLOPPY DISKETTES (RXA:)
software, the other (Logical-8) is compatible with Digital's
PDP-8 software. When in Logical-11 mode, 4 (single density)
or 2 (double density) sectors are grouped to form a 128
(36-bit) word logical block. The sector-to-sector
interleave factor is 2; the track-to-track skew is 6. When
in Logical-8 mode, 3 physical sectors (of either density)
are grouped to form a 96 (single density) or 192 (double
density) 36-bit word block. The sector-to-sector interleave
factor is 3; the track-to-track skew is 0. Physical tracks
74 thru 77 are not available when in Logical-8 mode; they
are reserved for use by diagnostics.
In physical addressing mode, a single sector of 32 or 64
36-bit words (depending on the diskette's density) is
transmitted by each IN, INPUT, OUT, or OUTPUT UUO. All
sectors, including track zero are accessible. The sectors
are sequentially numbered starting with zero.
Accessible tracks and buffer size
as a function of IO mode
Buffer size Contents
Density Addressing Tracks 36-Bit / 8-Bit Data words
------- ---------- ------ ------------------ ----------
Single Physical 77 32 words/128 bytes 64 16-bit
Single Logical-8 74 96 words/384 bytes 256 12 bit
Single Logical-11 76 128 words/512 bytes 256 16-bit
Double Physical 77 64 words/256 bytes 128 16-bit
Double Logical-8 74 192 words/768 bytes 512 12-bit
Double Logical-11 76 128 words/512 bytes 256 16-bit
Table 1
Note that, except in Logical-11 mode, incorrectly specifying
the density of a diskette may cause an unintentional address
check due to the differing buffer sizes.
Up to two RX20 controllers are supported per-CPU. The
controller names are RXA and RXB. Each controller can
support up to two RX02 drives. Thus, a system with the
maximum number of controllers and drives would include the
device names RXA0, RXA1, RXB0, and RXB1.
The diskette hardware provides the ability for each sector
to be marked "deleted" via use of a special "deleted data
mark" which the hardware can create and detect. TOPS-10
allows your program to create and detect deleted data marks
by manipulating the diskette's file status word (IO.PAR). A
sector which is flagged with a deleted data mark may still
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FLOPPY DISKETTES (RXA:)
contain valid data; the presence of a deleted data mark
does not alter the capacity of a sector. Every OUTPUT UUO
executed with IO.PAR set in the file status word will cause
a deleted data mark to be written with the data; every
OUTPUT UUO executed with IO.PAR clear in the file status
word will write only the data, and will erase any previous
deleted data mark. No Digital operating system makes use of
deleted data marks. However, some other vendors use them to
signify that a sector is available for use. Programs which
write diskettes for interchange with these vendors will use
deleted data marks as prescribed by the vendor. Programs
which write diskettes for interchange with Digital systems
should not write deleted data marks. Note that use of
deleted data marks as a storage allocation technique
requires your program to read a potentially large number of
sectors until a deleted data mark is encountered, then
rewrite the sector with new data. In contrast, Digital
systems use a bit table or directory structure to allocate
storage. This approach allows a sector to be allocated for
new data by simply checking the bit table or directory
(usually a few sectors in size) without an exhaustive scan
of the disk.
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FLOPPY DISKETTES (RXA:)
A floppy diskette may be operated only in buffered image
data mode (.IOIMG). The data mode must be specified by an
OPEN UUO, and preserved during any subsequent SETSTS UUOs.
The file status bits (which are read by GETSTS, and set by
OPEN and SETSTS UUOs) provide additional control over the
diskette. The following bits are defined for diskettes:
IO.DER Device error. Set when an error occurs
during an IO operation which is not
attributable to media.
IO.DTE Data error. Set when a media-attributable
error, such as a CRC (data parity) error,
occurs during an IO operation.
IO.BKT Block number too large. Set when IO is
attempted to a block number which is too
large for the selected device/density.
IO.WPS (Same as IO.BOT) Encodes the type of
interleaving desired when using logical IO
mode. When set, specifies that PDP-8
compatible mode is desired. When clear,
specifies that PDP-11 compatible mode is
desired. It is illegal to set IO.WPS when
using physical IO mode.
IO.PAR Set by the monitor when a deleted data mark
is encountered during an input operation.
Cleared by the monitor when a deleted data
mark is not encountered during an input
operation. Set by your program prior to an
output operation to cause a deleted data
mark to be written with each sector
transferred.
IO.DEN This two-bit field encodes the density and
addressing mode for the diskette. Setting
the high order bit specifies physical
addressing mode. Clearing the high order
bit specifies logical addressing mode.
Setting the low order bit specifies double
density mode. Clearing the low order bit
specifies single density mode.
IO.SIM When set, specifies that 2-sided media is
present in the drive. When clear, specifies
that 1-sided media is present. Like the
density bit of IO.DEN, IO.SIM is updated by
the monitor during each transfer. IO.SIM is
also used to determine the maximum number of
blocks which the device can store. (2-Sided
media has twice the number of
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FLOPPY DISKETTES (RXA:)
blocks/diskette for any given density.)
Digital hardware does not currently support
2-sided media.
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FLOPPY DISKETTES (RXA:)
Floppy diskettes respond to the following UUOs. Except as
noted, the calling sequence is the same as for any other
device.
DEVSIZ Returns the default buffer size and number of
buffers for a given IO mode. To ensure
compatibility with future releases, always use
DEVSIZ when allocating your own buffers. (Don't
"hardwire" buffer sizes.)
FILOP. IO functions perform the same functions as the
individual UUOs listed here.
OPEN Assigns a floppy diskette to a channel. See the
file status bits (above). Note that the initial
file status bits specified by an OPEN UUO
determine the size of buffers built by subsequent
INBUF, OUTBUF, IN, INPUT, OUT, or OUTPUT UUOs.
As a result, if you plan to use more than one IO
mode while a channel is OPEN, it is recommended
that you set the initial file status to the IO
mode wich requires the largest buffer size. This
will ensure that the largest possible buffers are
built by the monitor, and will prevent later IO
from over-writing the buffers' end.
Alternatively, you can build your own buffer
rings.
GETSTS Reads the current file status word.
IN,INPUT Reads one block or sector from the diskette.
OUT,OUTPUT Writes one block or sector to the diskette.
SETSTS Sets the current file status word.
USETI Specifies which block or sector is to be read
next. If USETI is not used, the first
block/sector read is zero. Subsequent IN or
INPUT UUOs will read the next consecutively
numbered block/sector.
USETO Specifies which block or sector is to be written
next. If USETO is not used, the first
block/sector written is zero. Subsequent OUT or
OUTPUT UUOs will write the next consecutively
numbered block/sector.
CLOSE flushes the buffer rings normally.
RELEAS Performs a CLOSE (if necessary), then disconnects
the channel from the device.
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FLOPPY DISKETTES (RXA:)
In addition to the UUOs listed above, the following DEVOP.
functions also control diskettes:
I. .DFRXS (=1) Read extended status
NOTE
This DEVOP. function provides
access to a hardware
maintenance function which may
be useful to the programmer in
limited circumstances. For a
description of the returned
status words, see the RX02
Technical Manual (SDC order
number EK-0RX02-TM-001).
Calling sequence:
MOVE AC,[XWD 4,addr]
DEVOP. AC,
error return
Normal return
addr: EXP .DFRXS ;function code
/ EXP UDX
< SIXBIT /devnam/ ;device specification
\ EXP channo
BLOCK 2 ;buffer for returned hardware status
;see the RX02 Technical Manual
Returns the following values in AC:
Normal return: IO status for device (GETSTS)
Error return: Reason for failure; one of:
DFACS% Address check (The argument list is not in user memory)
DFPRV% Device active (Operation illegal when device is active)
DFNLR% Value error (Format code was not SINGLE or DOUBLE)
DFNIA% Not initialized (IO.DER or IO.DTE set in file status)
DFDOL% Device offline
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FLOPPY DISKETTES (RXA:)
II. .DFFMT (=11) Format a diskette
Calling sequence:
MOVE AC,[XWD 3,addr]
DEVOP. AC,
error return
Normal return
addr: EXP .DFFMT ;function code
/ EXP UDX
< SIXBIT /devnam/ ;device specification
\ EXP channo
/ SIXBIT /SINGLE/ ;to format the diskette in single density
\ SIXBIT /DOUBLE/ ;to format the diskette in double density
Returns the following values in AC:
Normal return IO status for device (GETSTS)
Error return Reason for failure. See .DFRXS for error codes.
Diskettes purchased from Digital are pre-formatted for
either single density (RX01) or double density (RX02) use.
This DEVOP. allows you to format diskettes from other
vendors, and allows you to reformat a double density
diskette for single density use, and viceversa. Diskettes
purchased for single density use may not be usable for
double-density applications. After formatting a diskette,
we strongly recommend reading the entire disk, and mapping
any errors. A format operation takes about 15 seconds to
complete, and destroys any data previously on the disk.
Should a power failure, hardware malfunction, or operator
action interrupt a format operation, a diskette may result
which contains address marks of both densities. Such a
diskette is illegal, may result in improper operation, and
should be reformatted correctly.
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FLOPPY DISKETTES (RXA:)
III. .DFHCW (1002/2002) Read/Set the error counter
Calling sequence:
Read: MOVE AC,[XWD 2,addr]
DEVOP. AC,
error return
Normal return ;error counter stored in AC
addr: EXP .DFHCW ;function code
/ EXP UDX
< SIXBIT /devnam/ ;device specification
\ EXP channo
Set: MOVE AC,[XWD 3,addr]
DEVOP. AC,
error return
Normal return ;AC is not changed
addr: EXP .DFHCW+.DFSET ;function code
/ EXP UDX
< SIXBIT /devnam/ ;device specification
\ EXP channo
EXP new value ;(usually zero)
Error returns: AC contains an error code.
DFACS% Address check (argument list not in memory)
DFPRV% No privileges (set)
DFNLR% Value out of range (argument list too small)
DFNXD% Nonexistant device
Allows any program to read, and privileged programs to set
the error statistics word maintained by the monitor. This
word has the format:
_0___________________17_18___________________35_
|Number of hard errors | Number of soft errors !
------------------------------------------------
The monitor increments the number of soft errors once for
each IO operation which did not succeed on the first
attempt, but did succeed during monitor-initiated retries.
The monitor increments the number of hard errors once for
each IO operation which was not successful, even after
retrys. A given error is either a hard error, or a soft
error; no error is counted twice.
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FLOPPY DISKETTES (RXA:)
The error counter may be of use to a program which wishes to
move data from a sector which has a large number of soft
errors to a new sector, before the original sector becomes
unreadable. A program may also wish to inform the user when
a diskette should be replaced.
If a nonprivileged program wishes to use error information
based only on its use of a diskette, it must read the error
counter when it starts operation, and again when it
finishes. The difference between the two values reflects
that program's use. Privileged programs may simply clear
the error counter when they start. The distributed software
never resets the error counter, and the monitor makes no use
of the statistics accumulated. However, some installations
may reset the error counter on a MOUNT command, and log the
value on DISMOUNT in order to maintain a record of an
individual floppy's performance.
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