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this, processing must be interrupted to allow the hardware scan instruction to service the lines
(3 word times per line). The interruption must be timed so that from the end of. one scan cycle to
the end of the next scan cycle, the total elapsed time is less than one bit time.
Although the above only discussed the bit time for serial bit stream transmission buffers, the
scanning and processing of character and word buffers follow the same rules.
ADDRESSING MEMORY
There are six possible addressing modes on the DATANET-30.
These modes were specially
designed to solve the particular memory addressing problems involved in handling communication
lines.
Direct Access to a Program Bank
The instruction address specifies a memory location within the 1024 memory locations (program
bank) in which an instruction is located. Two numbers, 0-1, are in the mode number because the
instruction address is really only 9 bits (512 locations), but the size of a program bank is extended
to 1024 by using the low order bit of the addressing mode as part of the instruction address. This
mode is used 'for branching to instructions which are in the same subdivision of the program, or,
for accessing constants or working storage not needed in other program banks.
Direct Access to the Common Data Bank
The instruction address specifies a location in the common data bank, the first 512 memory
locations.
The location of an instruction is not significant. This mode is used for constants,
variables, and subroutine linkages which are common to parts of the program located in more
than one program bank. Any address in memory has direct access to these locations.
Direct Access to a Channel Table
The instruction address specifies the starting location of a channel table, i.e., an array of words
with one word for each input/ output channel. The true starting address must be modulo 16. When
the instruction is converted from its symbolic form into binary by the assembler, the true
address is divided by 16 and becomes the instruction address. When the instruction is executed,
the DATANET-30 effectively multiplies the instruction address by 16 to recover the true starting
address of the array.
The contents of the
C
register select the word in the array which corre-
sponds to the channel currently specified by the
C
register. This mode is used for various items
which exist for each channel, such as channel type, number of characters received so far,
where the next word is to be stored for a received message, etc.
Indirect Addressing
The remaining modes use indirect addressing to specify the actual location. With indirect
addressing, some location, such as ALPHA, is specified by an instruction and the actual location
used is determined by the contents of ALPHA. Ordinary indexing is possible when using inqirect
addressing.
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