Motorola M68000 User's Guide download pdf (Page 209)

The 68020 195

31 28 23 15 0

Base- *

+ $04

+ $08

+ $0C

+ $10

Figure 18 Module descriptor format (Courtesy of Motorola, Inc)

immediate operand specifying the number of arguments being passed to

the module. These arguments are passed on the stack. Before the CALLM

is executed, they must be pushed on the stack. Figure 18 shows the layout

of a module descriptor. The OPT field is restricted to two values: 000

indicates that arguments are passed on the stack, 100 indicates that a

pointer to the arguments will be provided on the stack. Only two values

for the type field are used: 00 indicates that no change in access level is

desired, 01 indicates that there may be a change in access rights. If type

01 is indicated, the called module is allowed to have a stack area that is

independent of the caller s stack.

The access level field is passed to the external hardware to provide

information for a possible change in access rights. The module data area

pointer contains the address of the called module s data area. This is

normally the value that will be loaded into the stack pointer. The module

entry word pointer is the address of the module’s entry word. This is

a special word that precedes the first instruction of the module. Figure

19 show the layout of this word. It merely specifies an address or data

effect is that the module data pointer is ignored. This is because SP is

overwritten with a new value following the execution of the CALLM.

Figure 20 shows the module call stack frame.

The RTM instruction has the following form:

RTM Rn

14 13 12 11

10 9 8

6 5

4 3

1 0

D/A | Register | 0

o !

0 | 0

I o I o

I 0

Operation Word of First Instruction

Figure 19 Module entry word. (Courtesy of Motorola, Inc)

Opt | Type | Access Level | (Reserved, Must be Zero)

Module Entry Word Pointer

Module Data Area Pointer

Additional User-Defined Information

1 2 ... 204 205 206 207 208 209 210 211 212 213 214 ... 255 256

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Motorola M68000 User's Guide Page 209

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