Motorola M68000 User's Guide download pdf (Page 211)

The 68020 197

MOVE.W SEM,DO

LOOP: MOVE.W DO,Dl

ADDQ.W #1,D1

CAS.W DO/DlfSEM

BNE LOOP

The CAS2 instruction operates in a similar manner to the CAS except

that it has additional operands, and two compares are performed instead

of just one. Both comparisons must show a match for the update registers

to be stored in the destination addresses. The destination addresses must

be specified using register indirect addressing with either a data register

or an address register. If one or both of the compares fails, the

destination operands are copied to the compare registers just as in the

case of the CAS. The CAS instruction allows byte, word and longword

operations. The CAS2 is restricted only to word or longword operations.

CHK2 The CHK2 instruction is an extended version of the CHK instruc

tion. CHK2 will check a data value in either a data register or an address

CHK2I.<size>] <ea>,Rn

<size> = B, W, L

The effective address must be to a pair of memory operands. The first is

the lower bound. It is a byte, word, or longword. The second is the upper

bound. This is located at the next byte, word, or longword in memory

above the lower bound. If the register, Rn, is less than the lower bound or

greater than the upper bound, an exception is generated. The exception

vector is the same as for the CHK instruction. The CHK2 instruction can

be used for both signed and unsigned operands.

CMP2 The CMP2 instruction is very similar to the CHK2 instruction. It

has the same general form:

CMP21.<size>] <ea>,Rn

<size> = B, W, L

The operands are identical to the CHK2 instruction. The difference is

that this instruction does not generate an exception if the register is out

of bounds; instead, the condition codes are used to indicate the success

or failure of the operation. The carry condition is set if the register value

is out of bounds. The zero condition is set if the register value is equal

to either the upper or lower bound. Here is how to test for whether the

value in register DO is greater than 100 or less than —100:

CMP2.L BOUNDS,DO

BOUNDS: DC.L -100,100

Coprocessor Support Instructions The 68020 provides direct coproces

sor support. A coprocessor is a special purpose chip that operates in con

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Motorola M68000 User's Guide Page 211