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Condition Testing

Often, the conditions that govern loops and conditional statements can be expressed simply as a check on the value of one of the CCR flags:

bne, beq, bmi . . .

In many cases however, we need to do more work to evaluate a condition:

while count !=5 do

. . .

endwhile

1BA3 – G Lacey – Lecture 17

CMP

In this case, we must subtract 5 from count and then check the Z flag (note: we don’t want to alter count):

while

move.w #$05,d1

sub.w d0,d1 * while count!=5

beq endwhile

...

endwhile

...

As this operation is so common in 68000 programming, an instruction is provided which does exactly that; the cmp instruction.

1BA3 – G Lacey – Lecture 17

CMP

CMP subtracts the source from the destination, sets CCR accordingly but does not store the result.

while

cmp.w #5,d0 * while count!=5

beq endwhile

...

endwhile

...

The CCR flags are set as if the instruction were:

sub.w $5,d0

1BA3 – G Lacey – Lecture 17

Greater than and Less than comparison

Greater than and less than comparison can be extracted from a combination of CCR flags:

1BA3 – G Lacey – Lecture 17

bhi and bge

Examples:

cmp.b #5,d0 * Unsigned

bhi higher -> branch if d0 is higher than 5

cmp.b d0,d1 * Signed

bge greater or equal -> branch if d1 d0

1BA3 – G Lacey – Lecture 17

Example (bhi) Unsigned:

bhi will branch if C = 0 and the Z = 0 i.e. !C & !Z

move.w #$10,d0

cmp.b #$11,d0

bhi higher

...

higher

d0 -> 0000 1010

test value -> 0000 1011-

1111 1111

N = 1; B = 1 -> C =1; Z = 0

!C & !Z = !1 & !0 = 0 & 1 = 0

-> is not higher

-> branch is not taken

1BA3 – G Lacey – Lecture 17

Example (bls) Unsigned:

bls will branch if C = 1 or Z = 1 i.e. C | Z

move.w #$10,d0

cmp.b #$11,d0

bls lower_s

...

lower_s

d0 -> 0000 1010

test value -> 0000 1011-

1111 1111

N = 1; B = 1 -> C =1; Z = 0

C | Z = 1 | 0 = 1

-> is lower or same

-> branch is taken

1BA3 – G Lacey – Lecture 17

Example (if):

- Slightly more complicated IF condition
- If the byte in $2000 is a valid BCD then store 1 in $2001 otherwise store a 0.
- BCD -> Values of each nibble < 10
- Binary - Coded Decimal (BCD)

1BA3 – G Lacey – Lecture 17

Solution in English

- Isolate the lower 4 bits.
- Isolate the upper 4 bits.
- If both values < 10
- Then the result is 1
- Otherwise the result is 0

1BA3 – G Lacey – Lecture 17

Pseudo-Code

lower = ($2000);

upper = ($2000);

lower = lower & %00001111

upper = upper & %11110000

upper = upper >> 4;

if (upper < 10 AND lower < 10)

result = 1;

else

result = 0;

endif;

($2001) = result;

We can have arbitrary complexity in the condition.

1BA3 – G Lacey – Lecture 17

Pseudo-Code converted to Assembly

move.b $2000,d0 * get lower

move.b d0,d1 * get upper

and.b #$0f,d0 * lower 4 bits

and.b #$f0,d1 * upper 4 bits

lsr.b #4,d1

cmp.b #9,d0 * if (lower<10 bhi ELSE

cmp.b #9,d1 * and upper<10) bhi ELSE

move.b #1,d2 * result = 1

bra ENDIF * else

ELSE

move.b #0,d2 * result = 0

ENDIF

move.b d2,$2001 * store result

1BA3 – G Lacey – Lecture 17

Test Condition, Decrement and Branch

a = 10;

b = 0;

do {

b = b + c;

a = a -1;

} while (a >= 0);

result = b;

This is a very common construct in programming. The MC68332 provides an instruction which implements the decrement and the condition in one: DBcc

1BA3 – G Lacey – Lecture 17

Example

move.w #10,d0 * a = 10;

move.w #0,d1 * b = 0;

doloop * do {

add.w d2,d1 * b = b + c;

dbra d0, doloop * } while (--a>=0)

move.w d2,$2000 * result = b;

1BA3 – G Lacey – Lecture 17

Note:

- The dbcc command allows the use of 2 loop guards at the same time:
- The Condition
- The Counter Value

- In the above example a dbra was used, as the loop was guarded be the count register d0 only

1BA3 – G Lacey – Lecture 17

Example

An example of both conditions (the counter and the condition test) being used simultaneously is the following:

move.w #10,d0 * a = 10;

move.w #0,d1 * b = 0;

DOLOOP * do {

add.w d2,d1 * b = b + c;

dbvs d0, DOLOOP * } while (--Cnt>=0

* AND !Overflow)

move.w d2,$2000 * result = b;

The dbvs above may be read as “decrement and branch until overflow set or register value less than 0”.

1BA3 – G Lacey – Lecture 17

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