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190 Assembly Language Programming for the 68000 Family

rect mode. The new 68020 modes make this operation a thing of the past.

Figure 17 shows the calculation of the effective address for the versions

using an address register. The versions of these modes for use with the

program counter are identical if the PC is substituted for the address reg

ister. In this case the memory address pointed to by the PC is that of the

first extension word. The two versions of the addressing mode are very

similar. The major difference is whether post-indexing or pre-indexing is

used.

The assembler syntax for these forms are:

(tbd,An],Rn.<size>*<scale>,od) post-indexed

([bd,An,Rn.<size>*<scale>],od) pre-indexed

([bd,PC],Rn.<size>*<scale>,od) post-indexed

([bd,PC,Rn.<size>*<scale>],od) pre-indexed

With the post-indexed versions, the memory location calculated by

adding An or PC to bd is used as the memory indirect location. This

is the memory location containing the pointer. The index register, appro

priately scaled, is added to this pointer as well as the outer displacement,

od. This final value is the effective address. With the pre-indexed versions

the sequence is slightly modified. The location of the memory indirect

pointer is determined by adding the address register or PC to the base

displacement, bd, and the scaled index register. The outer displacement

is added to the pointer to compute the final effective address. All four

of the specified values are optional. This actually provides 16 different

combinations.

Only the most advanced assembly language programmers will need

these advanced addressing modes. However, the pre-indexed versions are

especially useful when you have an array of pointers. You might keep this

in mind as you start programming with the 68020.

Instruction Extensions

The extensions to existing instructions generally fall into two cate

gories: increase in the size of allowable displacements, and increased

functionality.

The 68020 allows a full 32-bit displacement to be used with the branch

instructions BRA, BSR, and Bcc. Formally, the addressing range was

restricted to a 16-bit displacement. This extension allows us to reach

anywhere in memory with these instructions.

The CHK instruction will now operate with both word and longword

sources. Recall that the CHK instruction checks the contents of an address

specify the size of the bound using the usual method of appending .W or

.L to the instruction mnemonic.

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