Motorola M68000 Betriebsanweisung PDF herunterladen (Seite 202)

188 Assembly Language Programming for the 68000 Family

MOVE.L #1,D0

MOVEC DO/CACR

does the job nicely. The other bits can be manipulated in a similar manner.

Remember, MOVEC is a privileged instruction, so this operation can’t be

accomplished when in user mode. The cache can be enabled or disabled

at will.

The F bit will freeze the cache. The cache will still operate, but its

contents will not change. This is useful for certain types of applications

when a function is being emulated. The C and CE bits are used to clear

the entire cache or just a particular entry. The cache should be cleared

every time the the contents of program memory are changed. If program

memory is changed without the cache being cleared, the contents of

the cache will not reflect the actual contents of memory, and errors in

execution may result. This operation would normally be performed by

the operating system when it loads a new program. To clear a specific

entry we place the cache index value in the CAAR. The cache index value

is determined by using bits 2 through 7 of the corresponding memory

address. This value is placed in bits 2 through 7 of the CAAR. It doesn’t

matter what the other bits are, they are ignored. The CAAR is structured

like this:

31 8 7 2 1 0

INDEX

This last operation is provided only for the most sophisticated applica

tions. If a specific memory address in program space is modified, only

the specific cache entry that corresponds to it need be modified; the en

tire cache does not have to be cleared. Since self-modifying programs

have fallen out of vogue, it is unlikely that clearing a specific entry has

any value except for very sophisticated machine emulation applications.

Additional Addressing Modes

The 68020 provides six additional addressing modes besides the 12

that already exist for the 68000. These new addressing modes form two

groups of three. The operation of the two groups is very similar. The

first group extends the 68000’s address register indirect with index mode.

The second group extends the 68000’s program counter with index. You

will recall from Chapter 6 that these two modes are quite similar. The

program counter with index mode functions exactly like the address

the program counter.

1 2 ... 197 198 199 200 201 202 203 204 205 206 207 ... 255 256

Keine Kommentare

Motorola M68000 Betriebsanweisung Seite 202