Motorola M68000 Betriebsanweisung PDF herunterladen (Seite 73)

Conditional and Arithmetic Instructions 59

destination register. For example, the addition of the following unsigned

8-bit binary numbers results in a sum that will not fit in an 8-bit destination

(register or memory byte).

11101001

+ 10110111

110100000

The carry of a bit from the most significant (high-order) bit position has

no place to go. Actually, this event causes the carry bit (C) to be set. If

there was no carry, the carry bit would be reset.

In order to determine if the result of an unsigned addition is valid, the

setting of the carry bit must somehow be tested. Fortunately, the 68000

provides a group of instructions that allow the testing of each of the bits

of the CCR individually and in various combinations. These instructions

all have the form of a

conditional branch. A conditional branch is similar

to a JMP instruction except that for the former the jump is not taken

unless a particular condition is true. This condition corresponds to the

value or values of one or more of the bits of the CCR. If the branch is

not taken then the next sequential instruction is executed. The branch on

carry set instruction, BCS, provides a conditional branch based on the

setting of the carry bit. If the carry bit is set (binary one) then the branch

is taken. The following program illustrates the use of the BCS instruction

to validate the result of an unsigned addition.

HOVB.L NUH1/D0

ADD.L NUM2,D0

BCS INVALID

RESULT IS OK

INVALID: . RESULT EXCEEDS REGISTER SIZE

The label used with the BCS instruction, and with all the conditional

branches, must be to a location that is within approximately plus-or-minus

32,768 bytes of the location of the instruction itself. As mentioned in

Chapter 4, this restriction is also present with the DBRA instruction. It is

usually difficult to tell how many instructions will represent 32,768 bytes.

It is best to let the assembler do the work for you. But let’s suppose

that you really must perform a conditional branch to a location that is

greater than plus or minus 32,768 bytes. What can you do to get around

this problem? One simple solution is to perform the conditional branch

to a JMP instruction that is “close” to the conditional branch and then

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Motorola M68000 Betriebsanweisung Seite 73