Motorola M68000 User's Guide download pdf (Page 88)

74 Assembly Language Programming for the 68000 Family

word. This is similar to the technique used with immediate addressing,

but these words are an address and not the data itself.

The absolute short mode only provides a single extension word to the

instruction. This word has a range of from 0 to FFFF in hexadecimal.

The 68000 takes this word and sign-extends it to 32 bits. This means that

the sign bit is copied into the high-order 16 bits. Therefore the range of

an absolute short address is

0000000016 - 00007FFF16 and

FFFF800016 - FFFFFFFF16

An address between 0000800016 and FFFF7FFF16 does not lie in this range

and can not be accommodated by the absolute short addressing mode.

If you have a smart assembler and are specifying an address in the

short range, the assembler can use this mode and save a word of memory.

Some assemblers always use the long absolute mode. I should mention

that on the majority of systems, even with a smart assembler, absolute

short mode may never be used. This is because you will normally be

specifying addresses by their symbolic names. These addresses are not

really absolute in the sense that they are pinned down to specific memory

locations. Most programming environments use some form of program

relocation. This can be accomplished at the time of linking, or at the

time the program is loaded for execution, or at both times. Therefore,

the assembler can’t guarantee that the final address will be in range of

the short addressing mode. Your only chance to use this mode is if you

specify a numeric address explicitly such as

MOVE.L $100,DO

If your assembler is smart, it will use absolute short. One way to find out

is to look at the assembly listing and count the total number of words

generated for that instruction. If there are two, then absolute short was

used.

Address Register Indirect

This is an addressing mode that has not been introduced in earlier

chapters. When we write an assembly language program, we normally

use a symbol to specify the location of data in memory. It is the job of

the assembler to assign memory locations to each of these symbols. The

68000 CPU doesn’t operate with symbolic addresses; rather, it requires

the actual memory address. If we somehow had a method of obtaining

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Motorola M68000 User's Guide Page 88