Accelerator Manual (G06.24+, H06.03+)

Preparing Your Program for TNS/R Systems
Accelerator Manual—527303-002
2-10
Odd-Byte References
Odd-Byte References
When a language compiler applies a TNS word instruction to an operand with an odd-
byte address, run-time results are unpredictable. The instruction might do any, or any
combination, of:
•
“Round down” the misaligned address (as the TNS CISC processors always did)
•
Use the misaligned address without rounding it down
•
Cause the instruction to fail
The TNS instruction’s misalignment behavior depends on:
•
Operand size (16-bit, 32-bit, or 64-bit)
•
Execution mode
In TNS mode or accelerated mode, the action also depends on:
°
TNS opcodes such as QCMP
Although 64-bit integers with odd-byte addresses usually are not rounded
down, they can be (QCMP is one such example).
°
Contextual optimizations
°
How the instruction's result is used by later instructions
•
The SCF attribute TNSMISALIGN (see Misalignment Handling on page 2-15)
The varying behaviors and symptoms that misalignment errors cause can be confusing
for the following reasons:
•
A source program can produce erroneous TNS object code that seems to execute
correctly until you change the source program in an apparently innocent way.
•
The program might behave differently in TNS mode than it does in accelerated
mode. (Neither mode is better for avoiding alignment problems.)
•
The debugger might display a different value for a variable than the program sees,
because the debugger never “rounds down” addresses.
•
The program might “round down” addresses for storage operations but not for
subsequent retrieval operations of the same misaligned operand (or the reverse).
TNS/R systems do not support the same odd-byte references to doublewords (4-byte
units) or quadruplewords (8-byte units) as TNS systems do. Odd-byte references are
made when the least significant bit (bit 31) of an extended pointer is set.
•
TNS systems ignore bit 31 of an extended pointer and fetch or store data starting
from the byte prior to the addressed one to ensure that the address is on a word
boundary. This behavior is probably not what you intended.
•
On TNS/R systems, odd-byte references to doublewords or quadruplewords are
unpredictable; programs might trap or they might continue executing. Extended
pointers to variables accessed as doublewords or quadruplewords must not have