Alpha Microsystems Eagle 100 Specifikace stáhnout pdf (Strana 85)

APPENDIX D

ROADRUNNER AM-174

PROGRAMMING INFORMATION

D.1ROADRUNNER AM-174 PROGRAMMING INFORMATION

The MC68040 processor used on the AM-174 Roadrunner board contains more internal

instruction cache (4096 bytes) than earlier MC68040-based processors. Increased

internal cache is one of the features which contributes to the improved performance of

the Roadrunner 040 board; however, if your software does not properly manage this

instruction cache, it could be adversely affected.

When the instruction cache is enabled—as it would be during normal system

operation—any memory location which is executed by the processor is loaded into the

instruction cache. This improves performance for those cases where the same location

is executed again, since the instruction will be read from cache instead of slower main

memory.

D.1.1The Problem and Why It’s A Problem

The problem arises when that same memory location, which has now been loaded into

cache, is modified. Because data operations—such as the MOV instruction—do not

update the instruction cache, when your program next executes the memory location, it

may read the old contents, which are still in the instruction cache, rather than pulling the

instruction from main memory where it was updated. This is known as self-modifying

code.

In this context, self modifying code is any code which is written to memory as data, such

as when read from disk to memory, and then executed as instructions. AMOS handles

the most common cases, such as reading data from disk or during the FETCH monitor

call, but cannot handle cases in your software where you are handling program overlays

or building instructions on the fly. Such code is often present in "trick" code like that

used to perform SSD-based software protection, although it may be present anywhere.

This issue was present on 68030 based systems, but the limited cache size on that

processor made it a much smaller problem. Tricks to flush the cache that worked on the

68030, such as forcing a context switch, will probably not work reliably on the 68040.

PDI-00172-60, Rev. A02

1 2 ... 80 81 82 83 84 85 86 87 88 89

Žádné komentáře

Alpha Microsystems Eagle 100 Specifikace Strana 85