Write - IBM 7074 General Information Manual

Write
A write command must always be preceded by a pre-
pare to write check, prepare to write format, or a
prepare to verify order. A write command, not pre-
ceded by one of these orders, is terminated with an
UNUSUAL END signal with no transmission of data to
the file control. (A no operation order can be inserted
between the prepare to ... and the write command.)
Write Format Track Operation
To write a format track, three conditions must be met:
1. The format switch must be in the
WRITE
position;
if not, no writing of the format track will take
place.
2. A prepare to write format order with a track ad-
dress of one of the tracks of the cylinder must
be received and normally terminated by the file
control.
3. The prepare to write format order must be fol-
lowed by a write command specifying a core
storage location that contains the field of special
characters that are used to write a format track.
During a format write operation, the 7631 will
legally accept only four characters: BCD 1,
BCD
2, BCD 3,
and BCD 4. For operation in the 6-bit mode, BCD l's
are used to write the special character of seven one
bits; BCD 2' s are used to write the special character of
seven zero bits. Seven bit positions are required for
each 6-bit character; the extra bit position is required
for a space bit used to separate characters. For opera-
tion in the 8-bit mode, BCD 3's are used to write the spe-
cial character of nine one bits; BCD 4's are used to write
the special character of nine zero bits. Nine bit posi-
tions are required for each 8-bit character; the extra bit
position is required for a space bit used to separate
characters.
A typical core storage layout for n records in both
the 6-bit mode and the 8-bit mode is shown in Figure
10. When organizing a data track in core storage, only
the information that is to be recorded must be pro-
vided for; however, when organizing a format track
Track Identification
iGap HAl Gap 2
HA2 X Gap RAI Y Gap
I
I
in core storage, the special BCD characters must be
provided to write every character position. This in-
eludes the HAl area, as well as all other addresses
and gap areas that are to be used to define the related
areas for the associated data tracks.
When laying out a format track, the following con-
siderations must be given:
Format Track Capacity: The maximum character
positions available when writing a format track is:
Six-Bit Mode: 2,869 character positions
Eight-Bit Mode: 2,234 character positions
These figures represent the total data track capacity
(2,840 character positions for 6-bit; 2,205 character
positions for 8-bit) plus HAl area (24 character posi-
tions) plus the HA2 gap (16 character positions), minus
11 character positions (space only, user does not pro-
vide characters for this area), that must be provided
for at the end of the track that are to be automatically
written by the 7631 File Control. Exceeding the track
capacity results in a format check.
Track Identification: This area immediately follows
the index point and corresponds to the HAl pre-
recorded data track area and consists of three parts:
Gap 1 must contain three BCD 4's; HAl must contain nine
BCD 3's; Gap 2 must contain a BCD 4 followed by ten
BCD 3's and a BCD 4. The track identification area is
always written in the 8-bit mode and will be the same
for all format track layouts.
Home Address Two (HA2): This area must contain
four more BCD 1's (6-bit mode) than the number of
characters assigned for HA2. The four character posi-
tions are needed for each record and each address
area to provide space for internal synchronization in-
formation. For example, if HA2 is a two character
home address identifier, then, six BCD 1's must be used
in this area. For operation in the 8-bit mode, BCD 3's
must be used instead of the BCD 1's.
X Gap: An X gap must precede every record ad-
dress (RA). This gap is made up of 12 BCD 2's or 12
BCD 4's, depending upon the mode of operation.
Record Address (RA): This area must contain four
more BCD 1's than the number of characters aSSigned
Gap
Record 1
YGap Record N
3
6-Bit
I
444 333333333
43333333333~
111111 222222222222 1111111111 211111111112 111----111
X
Gap
[RAN
222222222222 1111111111 211111111112 111----111 2
a-Bit
Number of BCD
Characters
(1,2,3, or 4)
444
3
333333333
43333333333~
9
12
Figure 10. Format Track Layout
333333 444444444444
6 Min 12
333333333::1 433333333334 333----333 444444444444 It3333333 433333333334 333----33J 4
10 Min
12
6 Min
12
10 Min 12 6 Min 1
Operations 13