Specifications

devices. Many devices will require less than sixteen status bits. Other devices
will require more than sixteen bits and therefore will require additional status
and control registers.
Device Function Bits-These three bits specify operations that a device iS
to perform. An example of one operation for a paper tape read&r is read
one character. For a disk one operation would be read a block of words from
memory and store them on the disk.
Memory Extension Bits-These two bits are resewed for future expansion.
They will allow devices to ‘use a full 18 bits to specify addresses on the bus.
Done Enable and Error Enable Bits-These two bits are independently Pro-
grammable. If bit 6 is set, an interrupt will occur as a reSUlt of a function
done condition. If bit 5 is set, an interrupt will occur as the result of an
error condition. This occurs when one or more of the error bits is sat to a
one. To initiate an interrupt routine to read from the paper tape reader,
the instruction
MOV #lOl, PRS
could be used. This sets bit 0 and bit 6 of the paper tape reader control and
status register. Setting bit 0 starts the read operation and setting bit 6
enables an interrupt to occur when the read operation’is complete.
Condition Bits--The CSR contains a DONE bit, a READY bit, or a DONE-
BUSY pair of bits, depending on the device. These bits are set and cleared
by the hardware, but may be queried by the program to determine the
availability of the device. For example, the teleprinter status register (TPS)
has a READY bit (7) that is cleared on request for output and then set when
output is complete. The keyboard status register (TKS) has a DONE-BUSY
pair (Bits 7 and 11) that distinguishes between no input (DONE = BUSY
= 0), input under way (DONE = 0, BUSY = 1). and input complete (DONE
= 1, BUSY = 0).
.
The DONE bit could be used to control an input loop for reading from- the
paper tape reader as follows:
LOOP: TSTB PRS ; test low byte of paper tape status register
BPL LOOP
; branch back if DONE bit (bit 7) is not set
Unit Bits-Some peripheral systems have more than one device per control.
For example, a disk system can have multiple surfaces per control and an
analog-to-digital converter can have multiple channels. The unit bits select
the proper surface or channel.
Error Bits--Generally there. is an individual bit associated with a specific
error. When more bits are required for errors, they can be obtained by ex-
panding the error section in the word or by using another status word.
Example of Control and Status Register
-The high-speed paper tape reader
control and status register (PRS) is as follows:
These bits may be read or set by instructions which use the appropriate
effective address. Bit 0 of the PRS is the function bit for reading one char-
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