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### HCS12 Arithmetic

Lecture 3.3

68HC12 Arithmetic

- Addition and Subtraction
- Shift and Rotate Instructions
- Multiplication
- Division

HCS12 code for 1+, 2+

; 1+ ( n -- n+1 )

ONEP

LDD 0,X

ADDD #1

STD 0,X

RTS

; 2+ ( n -- n+2 )

TWOP

LDD 0,X

ADDD #2

STD 0,X

RTS

HCS12 code for 1-, 2-

; 1- ( n -- n-1 )

ONEP

LDD 0,X

SUBD #1

STD 0,X

RTS

; 2- ( n -- n-2 )

TWOP

LDD 0,X

SUBD #2

STD 0,X

RTS

68HC12 Arithmetic

- Addition and Subtraction
- Shift and Rotate Instructions
- Multiplication
- Division

Bit

7

6

5

4

3

2

1

0

1

1 0 1 0 0 1 0 1

Logic Shift LeftLSL, LSLA, LSLB0

C

Bit

7

6

5

4

3

2

1

0

7

6

5

4

3

2

1

0

1

1 0 1 0 0 1 0 1

0

1 0 1 0 0 1 0 1

A

B

LSLD

Logic Shift RightLSR, LSRA, LSRB

C

Bit

7

6

5

4

3

2

1

0

0

1 0 1 0 0 1 0 1

1

C

Bit

7

6

5

4

3

2

1

0

7

6

5

4

3

2

1

0

0

1 0 1 0 0 1 0 1

1 0 1 0 0 1 0 1

1

A

B

LSRD

68HC12 Arithmetic

- Addition and Subtraction
- Shift and Rotate Instructions
- Multiplication
- Division

Dec

Hex

3D

x 5A

262 A x D = 82, A x 3 = 1E + 8 = 26

131 5 x D = 41, 5 x 3 = F + 4 = 13

157216 = 549010

61

x 90

5490

; multiply 8 x 8 = 16

=00004000 ORG $4000

4000 86 3D LDAA #$3D

4002 C6 5A LDAB #$5A

4004 12 MUL

product = $1572 is in D = A:B

31A4

x1B2C

253B0

4 x C = 30

A x C = 78 + 3 = 7B

1 x C = C + 7 = 13

3 x C = 24 + 1 = 25

31A4

x1B2C

253B0

6348

2220C

4 x B = 2C

A x B = 6E + 2 = 70

1 x B = B + 7 = 12

3 x B = 21 + 1 = 22

31A4

x1B2C

253B0

6348

2220C

31A4

0544D430

ORG $4000

4000 CC 31A4 LDD #$31A4 ;D = $31A4

4003 CD 1B2C LDY #$1B2C ;Y = $1B2C

4006 13 EMUL;Y:D = D x Y

Multiply Instructions

Note that EMUL and MUL are unsigned multiplies

EMULS is used to multiply signed numbers

Unsigned Multiplication

Hex

Dec

FFF8

x 0008

7FFC0

65528

x 8

524224

52422410 = 7FFC016

But FFF8 = 1111111111111000 can be a signed number

2’s comp = 0000000000001000 = $0008 = 810

Therefore, FFF8 can represent -8

Signed Multiplication

Dec

6410 = 0000004016

= 0000 0000 0000 0000 0000 0000 0100 0000

2’s comp = 1111 1111 1111 1111 1111 1111 0100 0000

= $FFFFFF40

-8

x 8

-64

Therefore, for signed multiplication

$FFF8 x $0008 = $FFFFFF40

and not $7FFC0

The EMULS instruction performs SIGNED multiplication

Signed Multiplication

; EMULS signed 16 x 16 = 32

=00004000 ORG $4000

4000 CC FFF8 LDD #$FFF8 ;D = $FFF8

4003 CD 0008 LDY #$0008 ;Y = $0008

4006 1813 EMULS;Y:D = D x Y

product = $FFFFFFC0 is in Y:D

Multiplication

Note that EMUL is a 16 x 16 multiply that produces

a 32-bit unsigned product.

If the product fits into 16-bits, then it produces the

correct SIGNED product.

68HC12 Arithmetic

- Addition and Subtraction
- Shift and Rotate Instructions
- Multiplication
- Division

A

C

EE BC2F

B28

9A

F

94C

63

Dividend = BC2F

Divisor = EE

Quotient = CA

Remainder = 63

A x E = 8C

A x E = 8C + 8 = 94

A

C

EE BC2F

B28

9A

F

94C

63

=00004000 ORG $4000

4000 CD 0000 LDY #$0000

4003 CC BC2F LDD #$BC2F

4006 CE 00EE LDX #$00EE

4009 11 EDIV;BC2F/EE = CA rem 63

Remainder in D

o

n

d

i

t

i

o

n

c

o

d

e

r

e

g

i

s

t

e

r

S

X

H

I

N

Z

V

C

Divisor may be too small11313 rem 85

EE FFBC2F

Quotient does not fit in Y

Overflow bit, V, will be set

o

n

d

i

t

i

o

n

c

o

d

e

r

e

g

i

s

t

e

r

S

X

H

I

N

Z

V

C

Note divide by zero sets

carry bit, C

N, Z, and V are undefined

Y and D unchanged

Remainder in D

Note symmetric division

Truncation toward zero

Sign of remainder = sign of dividend

Remainder in D

Note symmetric division

Truncation toward zero

Sign of remainder = sign of dividend

Symetric Division (truncation toward zero)

Floored Division (truncation toward minus infinity)

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