INSTRUCTION SET OVERVIEW
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5
5.3.1.8
Unaligned Memory Accesses
Unaligned memory accesses are performed by microcode. Microcode sequences the access into
smaller aligned pieces and merges the data as needed. As a result, these accesses are not as efficient
as aligned accesses. In addition, no bursting on the external bus is performed for these accesses.
Whenever possible, unaligned accesses should be avoided.
5.3.2
Miscellaneous Optimizations
5.3.2.1
Masking of Integer Overflow
The i960 core architecture inserts an implicit
syncf
before performing a call operation or
delivering an interrupt so that a fault handler can be dispatched first, when necessary. The
syncf
can require a number of cycles to complete when a multi-cycle integer-multiply (
muli
) or
integer-divide (
divi
) instruction was issued previously and integer-overflow faults are unmasked
(allowed to occur). Call performance and interrupt latency can be improved by masking
integer-overflow faults (AC.om = 1), which allows the implicit
syncf
to complete more quickly.
5.3.2.2
Avoid Using PFP, SP, R3 As Destinations for MDU Instructions
When performing a call operation or delivering an interrupt, the processor typically attempts to
push the first four local registers (pfp, sp, rip, and r3) onto the local register cache as early as
possible. Because of register-interlock, this operation stalls until previous instructions return their
results to these registers. In most cases, this is not a problem; however, in the case of multi-cycle
instructions (
divo, divi, ediv, modi, remo, and remi
), the processor could be stalled for many
cycles waiting for the result and unable to proceed to the next step of call processing or interrupt
delivery.
Call performance and interrupt latency can be improved by avoiding the first four registers as the
destination for a MDU instruction. Generally, registers pfp, sp, and rip should be avoided they are
used for procedure linking.
5.3.2.3
Use Global Registers (g0 - g14) As Destinations for MDU Instructions
Using the same rationale as in the previous item, call processing and interrupt performance are
improved even further by using global registers (g0-g14) as the destination for multi-cycle MDU
instructions. This is because there is no dependency between g0-g14 and implicit or explicit call
operations (i.e., global registers are not pushed onto the local register cache).
Содержание i960 Jx
Страница 1: ...Release Date December 1997 Order Number 272483 002 i960 Jx Microprocessor Developer s Manual ...
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Страница 25: ...1 INTRODUCTION ...
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Страница 35: ...2 DATA TYPES AND MEMORY ADDRESSING MODES ...
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Страница 47: ...3 PROGRAMMING ENVIRONMENT ...
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Страница 73: ...4 CACHE AND ON CHIP DATA RAM ...
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Страница 85: ...5 INSTRUCTION SET OVERVIEW ...
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Страница 111: ...6 INSTRUCTION SET REFERENCE ...
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Страница 195: ...INSTRUCTION SET REFERENCE 6 83 6 Opcode mov 5CCH REG movl 5DCH REG movt 5ECH REG movq 5FCH REG See Also LOAD STORE lda ...
Страница 233: ...7 PROCEDURE CALLS ...
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Страница 257: ...8 FAULTS ...
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Страница 291: ...9 TRACING AND DEBUGGING ...
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Страница 309: ...10 TIMERS ...
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Страница 325: ...11 INTERRUPTS ...
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Страница 369: ...12 INITIALIZATION AND SYSTEM REQUIREMENTS ...
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Страница 413: ...13 MEMORY CONFIGURATION ...
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Страница 429: ...14 EXTERNAL BUS ...
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Страница 469: ...15 TEST FEATURES ...
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Страница 493: ...A CONSIDERATIONS FOR WRITING PORTABLE CODE ...
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Страница 503: ...B OPCODES AND EXECUTION TIMES ...
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Страница 515: ...C MACHINE LEVEL INSTRUCTION FORMATS ...
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Страница 523: ...D REGISTER AND DATA STRUCTURES ...
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Страница 551: ...GLOSSARY ...
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Страница 561: ...INDEX ...
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