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service (two more bytes from the buffer), con-
tinuing regeneration. Thereafter, byte pairs are
retrieved from the buffer as required by the 2840
circuitry and the 2250
1
s being serviced.
When displays are being regenerated simultane-
ously for more than 2250, the 2840 alternately
checks each 2250 and associated 2840 circuitry to
determine whether it requires service and provides
this service as necessary. For example, if data
bytes are needed for 2250 No. 1 (Figure 13), the
contents of address register 1 are used to read two
bytes from the buffer: one from the ?ddressed buffer
location and one from the next sequential buffer loca-
tion. Also, the contents of address register 1 are
stepped by 2 in preparation for the next service se-
quence. The 2840 then determines whether 2250 No.
2 needs data bytes;
if
it does, the contents of address
register 2 are used to read two bytes from the buffer in
an operation similar to that described for 2250 No. 1.
During display regeneration, each byte pair read
from the buffer (except the second byte pair of a
four-byte order or the second and third byte pairs
of a six-byte order) is checked for an SM code in
the first byte. The presence of an SM code identi-
fies the byte pair as either a complete order
or the first byte pair of an order; the absence of an
SM code in the first byte identifies the byte pair as
data.
When the first byte for a specific 2250 contains the
SM code, the second byte (MC byte) is decoded, and
resulting signals are sent to the 2840 control section.
These signals set up the various conditions necessary
for proper interpretation of subsequent data or orders
for the 2250. The data for each 2250 is interpreted
as defined by the last order decoded for that 2250.
Buffer service requests for each 2250 are satis-
fied sequentially. The next two bytes read from the
buffer for 2250 No. 1 are either another orde.r, the
second two bytes of a four- or six-byte order, the
third two bytes of a six-byte order, or graphic or
character data. Operation with another order is
the same as just described. The second two bytes
of a four- or six-byte order will be either no-op'ed
(Enter 4-Byte No-Op) or passed to address register
1, the address register associated with 2250 No. 1
(Transfer Unconditional).
Each Graphic and Character mode order is normal-
ly followed by a series of data bytes. These bytes
are stored in consecutive buffer locations following
the associated order. Every byte pair containing
data is checked for an SM code in the buffer register;
therefore, orders can follow orders without inter-
vening data. When an SM is not found, the byte pair
is transferred to the data register associated with
the 2250 being serviced (for example, data register
1 when the data is for 2250 No. 1).
26
Once the byte pair is in the data register, the
specific Character mode or Graphic mode order
last decoded for the associated 2250 determines
how the two bytes will be used. Data bytes follow-
ing a Graphic mode order are transferred directly
to the appropriate 2250, whereas the data bytes
following a Character mode order are transferred
to the character generator.
The 2840 attempts to transfer bytes sequentially
to all 2250's, either directly from the data registers
or from the character generator. Byte transfer for
a specific 2250 does not occur when certain control
conditions exist (e.g. , light-pen detect).
If
byte
transfer for one 2250 halts, transfer continues for
other 2250's.
Each pass through the regeneration sequence for
a 2250 continues until a four-byte Transfer Uncon-
ditional order is decoded. This order is normally
used to branch from the last address in the buffer
area used by a 2250 to the first address of the area,
completing a loop for regeneration of the image.
In
this way, the display is repetitively rewritten, or
regenerated, from bytes in the same buffer area.
Regeneration continues automatically until stopped
by the channel program, by a parity check, by an End
Order Sequence order, or by a light-pen interrupt.
DISPLAY REGENERATION TIMING
The execution time available to display a flicker-
free image is 25 ms. An image that requires less
than 25 ms display time is held to a regeneration
rate of 40 cps by the Start Regeneration Timer order
so that a constant intensity level is maintained. An
image that requires more than 25 ms display time
has a regeneration rate of less than 40 cps, a rate
that may result in objectionable flicker.
The execution time of a display program is deter-
mined by the order execution time, the data display
time, and the buffer access time. Also affecting
execution are the number of 2250's operating simul-
taneously and the length of cable attaching each 2250
to the common 2840.
Because of the many variables involved, it is
impractical to attempt precise computations of
execution times. Instead, these paragraphs present
a qualitative description of system operation, follow-
ed by results for specific test cases. Execution
times for other images can be approximated by
interpolation.
All 2250' s attached to a common 2840 share the
2840 buffer cycles. This sharing is accomplished
with interleaved accesses to the buffer. Each order
requires one or more buffer accesses; the data for
each line or point requires one access for Incre-
mental mode and two accesses for Absolute mode,
(
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