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Operation Code Trap; Arithmetic Overflow And Divide Exception - RCA Spectra 70 Training Manual

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efficiently the overlap capabilities of a system con-
taining eight I/O Channels.
A communications device request, or a request for
control by the operator at the console typewriter also
generates an I/O Interrupt. Console request inter-
ruption is distinguished by the fact that the 27 bit of
the Standard Device Byte is set to (1)2'
Prior to entering the P2 state, the computer auto-
matically:
1.
Stores the state ofthe Condition Code Indicator,
The present value ofthe Condition Code is stored
in the 2 0 - 21 bits of the reserved HSM location
forty-three (2Bh6'
~~
The Condition Code Indicator is then set to (00) 2'
2.
Stores the identification (Trunk and Device
Number)
of the interrupting device in the
reserved HSM location forty-seven (2Fh6' The
Device Number is stored in the 2 0 - 2 3 bits, and
the Trunk Number is stored in the 24 - 27 bits.
BYTE (47)10
127 126 125124123\ 2 2 \ 21 120
~~
Trunk
Number
Device
Number
3.
Stores the Standard Device Byte for the Inter-
rupting device in the reserved HSM location
forty-six (2Eh6'
See page 53 for a desc ription of the Standard Device
Byte.
The P2 counter contains the address of the first in-
struction of a routine to be executed when interrupt
occurs. This routine tests the Condition Code (with
a Branch On Condition instruction). A setting of
(00) 2 indicates that interrupt had been caused by an
I/O device. The Trunk and Device Number have been
stored in a reserved area of HSM,
allowing the
routine to identify the device that caused the interrupt.
For example, if the Console Typewriter is Device
one on Trunk three, and the Interrupt button had
been depressed, then HSM location forty-seven would
contain:
Trunk
3
Device
1
10
OPERATION CODE TRAP
If an instruction is staticized in which the Operation
code is not one of the thirty-one legitimate codes, an
interrupt is initiated. This interrupt is called an
Operation Code Trap.
Prior to entering the P2 state, the computer auto-
matically:
1.
Stores the state of the Condition Code Indicator
in the 2 0 - 21 bits oflocation forty-three (2Bh 6'
2.
Stores the illegal operation code that caused the
interrupt in the reserved HSM location forty-
two (2A)16'
BYTE (42)10
The two high-order bits of the Operation Code in-
dicate the length of the instruction.
00
01 or 10
11
two-byte instruction
four-byte instruction
six-byte instruction
3.
Sets the Condition Code to (01)2' The interrupt
routine tests the Condition Code. A setting of
(01)2 indicates that the interrupt was caused by
an illegal operation code in the instruction pre-
viously staticized in the PI state. Depending on
the situation, the illegal operation could actually
be an error, or an intentional interrupt. In the
latter case, the interrupt could simulate an in-
struction that is not part of the 70/25 order code.
For example, the 70/45 operation code (4E)16
for Convert Decimal would cause an interrupt
on the 70/25.
However, the decimal conversion
could be simulated by instructions in the P2
state.
ARITHMETIC OVERFLOW AND DIVIDE EXCEPTION
A carry out of the high-order position of the first
operand during the execution of an Add Decimal
(FAh6 or a Subtract Decimal (FBh6 instruction
causes interrupt. If the operands of a Divide Decimal
(FD) 16 operation are not properly edited, an inter-
rupt occ urs .
Hardware stores the state of the Condition Code in
the 20 - 21 bits of reserved location forty-three
(2Bh6' and resets the code to (10)2' before trans-
ferring to the Interrupt State.

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Spectra 70/25