•• Arithmetic operations combine the contents of ACCU 1 with those
of ACCU 2, write the result to ACCU 1 and transfer the contents
of ACCU 3 to ACCU 2 and the contents of ACCU 4 to ACCU 3
(stack drop). In 16-bit fixed point arithmetic, only the low word or
ACCU 3 is transferred to the low word of ACCU 2 and the low
word of ACCU 4 to the low word of ACCU 3.
•• When a constant is added (ADD BF/KF/DH) to the contents of
ACCU 1, the accumulators 2, 3 and 4 are not changed.
Condition codes
STEP 5 operations either set or evaluate condition codes. The condition
codes are written to a condition code byte. Two groups of condition codes
can be distinguished: condition codes of digital operations (word
condition codes - bits 4 to 7 in the condition code byte) and condition
codes from binary and executive operations (bit condition codes - bits 0 to
3 in the condition code byte). You can see how the various condition
codes are influenced or evaluated by STEP 5 operations be referring to
the operation list /1/.
You can display the condition code byte on a programmer using the
"STATUS" online function (refer to Section 11.2.3). The byte has the
following structure:
Word condition codes Bit condition codes
CC 1 CC 0 OV OS OR STA RLO
ERAB
Bit 7 6 5 4 3 2 1 0
Bit condition codes
•• ERAB First bit scan
A logic operation sequence containing binary operations always
begins with the first bit scan, following which a new RLO is
formed. The bit condition code
ERAB = 1 is then set. While the
remaining logic operations in the sequence are being performed,
ERAB remains set to 1 and the RLO cannot be changed by these
logic operations.
The active sequence of logic operations is terminated by a binary
set/reset operation (e.g. S Q 5.0). The set/reset operation sets
ERAB to 0; the RLO can be evaluated (e.g. by RLO-dependent
operations) but can no longer be combined logically. The next
binary logic operation following a binary set/reset operation is
once again a first bit scan.
STEP 5 Operations with Examples
CPU 948 Programming Guide
3 - 16 C79000-G8576-C848-04
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