1. ME 4447/6405 Microprocessor Control of Manufacturing Systems and
Introduction to Mechatronics
Instructor: Professor Charles Ume
Lecture #10

2. Relative Addressing

3. Relative Addressing
Relative addressing is used for branch instructions only.
Three types of branch instructions:
Short Branches – offset signed 8-bit number (-128 to 127)
Long Branches – offset signed 16-bit number (-32,768 to 32,767)
Bit Condition Branches – offset signed 8-bit number (-128 to 127)
Short Branch instructions are 2 bytes in length.
First byte contains opcode
Second byte contains address offset
BEQ Loop 27 offset (I byte)

4. Relative Addressing Long branch instructions are 4 bytes in length
First two bytes contain opcode and postbyte
Second byte contains 16-bit signed address offset
LBEQ Loop MSB LSB  offset value
Bit condition branch instructions are 4 to 6 bytes in length depending on addressing mode
BRSET $0800 #$05 Loop 1E byte offset value
BRSET $15, Y #$EA Label 0E 4F EA 1byte offset value
Offset is calculated by subtracting contents of Program Counter from destination address. (Note: The Program Counter contains the memory location of the next instruction)
If branch condition is true, then add offset to program counter

5. Example Program for Relative Addressing Mode
ORG $ Address Opcode Postbyte Operand
ABA $
: :
BACK LDAA #$ $106A
NEGA $106C
ADDA $1A00 $106D BB A00
BEQ FRONT $
ADDA $ $ B
BNE BACK $
FRONT NOP $ A7
SWI F
END
Offset ?
Offset ?

6. Example 1 First calculate address offset for forward branch
ORG $ Address Opcode Postbyte Operand
ABA $
: :
BACK LDAA #$ $106A
NEGA $106C
ADDA $1A00 $106D BB A00
BEQ FRONT $
ADDA $ $ B
BNE BACK $
FRONT NOP $ A7
SWI 3F
END
Offset ?
Offset ?

7. Example 1 Branching Forward
Offset is positive if branching forward

8. Example 1
Address of BEQ instruction and Address of where to jump to (NOP)
ORG $ Address Opcode Postbyte Operand
ABA $
: :
BACK LDAA #$ $106A
NEGA $106C
ADDA $1A00 $106D BB A00
BEQ FRONT
ADDA $ $ B
BNE BACK $
FRONT NOP A7
SWI 3F
END
$1070
Offset ?
Offset ?
$1080

9. Step2: Subtract result from destination address $1080-$1072 = $0E
Example 1 Branching Forward Continued
In Hexadecimal
Step1: Add $02 to starting address $1070+$02 = $1072
to determine contents of Program Counter
Step2: Subtract result from destination address $1080-$1072 = $0E
Address Offset is : $0E $
+ $ $
1’s COMP
2’s COMP +$
= $0E

10. Example 1 Calculate address offset for backward branch
ORG $ Address Opcode Postbyte Operand
ABA $
: :
BACK LDAA #$ $106A
NEGA $106C
ADDA $1A00 $106D BB A00
BEQ FRONT $ E
ADDA $ $ B
BNE BACK $
FRONT NOP $ A7
SWI 3F
END
Offset ?

11. Example 1 Branching Backward
Offset negative if branching backward

12. Example 1
Address of BNE instruction and Address of where to jump to (LDAA)
ORG $ Address Opcode Postbyte Operand
ABA $
: :
BACK LDAA #$
NEGA $106C
ADDA $1A00 $106D BB A00
BEQ FRONT $
ADDA $ $ B
BNE BACK
FRONT NOP $ A7
SWI 3F
END
$106A
$1074
Offset ?

13. Example 1 Branching Forward Continued
Note: In Binary $
+ $ $
1’s COMP
2’s COMP
+$106A
= $F4

14. Example 1 Branching Backward Continued
In Hexadecimal
Step1: Add $02 to starting address $1074+$02 = $1076
to determine contents of Program Counter
Step2: Subtract result from destination address $106A-$1076
But result will be negative so:
Step 3: Switch order and subtract $1076-$106A = $0C
Step 4: Take 1’s complement $0C -> $F3
Step5: Take 2’s complement $F3 + $01 = $F4
Address offset will be: $F4

15. Example 1 Example Program assembled completely
ORG $ Address Opcode Postbyte Operand
ABA $
: :
BACK LDAA #$ $106A
NEGA $106C
ADDA $1A00 $106D BB A00
BEQ FRONT $ E
ADDA $ $ B
BNE BACK $ F4
FRONT NOP $ A7
SWI 3F
END

16. Address Opcode Postbyte Operand
Long Branch Instructions
Long branch instructions are equivalent to short branch instructions, except offset is signed 16-bit number [-32,767 to 32,768]
Example: Assume distance from branch instruction to destination is greater than 127 memory locations. Must use long branch instructions:
Short Branch:
BNE FRONT $ A
: :
FRONT NOP $ A7
Long Branch:
LBNE FRONT $
FRONT NOP $ A7
Address Opcode Postbyte Operand

17. Example Branch Instructions
Branch if bit(s) clear (BRCLR)
Example: Branch to DOG when bit 0 and bit 2 are clear in data contained at address $19
BRCLR $19 #$05 DOG
Assume $19 contains #$63
#$63 =
#$05 = (MASK) will not branch to DOG
Assume $19 contains #$62
#$62 =
#$05 = (MASK) will branch to DOG
(Note: BRCLR may be used in extended addressing mode.)
Can also be written as:
LDY #$0019
BRCLR $00,Y #$05 DOG

18. Example Branch Instructions
Branch if bit(s) set (BRSET)
Example: Branch to DOG when bit 0 and bit 2 are set in data contained at address $20
BRSET $20 #$05 DOG
Assume $20 contains #$63
#$63 =
#$05 = (MASK) *will not branch to DOG
Assume $20 contains #$65
#$65 =
#$05 = (MASK) *will branch to DOG
Can also be written as:
LDX #$0020
BRSET $00,X #$05 DOG

19. Branch Instructions from Programming Reference Guide

20. Branch Instructions from Programming Reference Guide (continued)
(Note: BRCLR and BRSET boxed in Red)

21. Special Instructions Bit Clear Example: BCLR $34 #$04
Assume $34 contains #$44
#$44 =
#$04 =
Result: $34 will now contain
Alternative:
If index register X contains #$0034, we can use the following instruction
to perform the same operation as above.
BCLR $00,X #$04

22. Special Instructions Cont’d
Bit Set
Example:
BSET $00,X #$01
Assume the content of index register X is #$0045 and assume that $45
contains #$60.
#$60 =
#$01 =
Result: $45 now contains
Alternative:
We can use the following instruction to perform the same operation as
above.
BSET $45 #$01

23. Special Instructions Cont’d
BITA
This instruction is similar to ANDA except that the result of this
operation is not stored in accumulator A.
Example:
BITA #$7C
BNE LOOP
Assume accumulator A contains #%
#$7C = #%
Accumulator A will still contain #% after above program is executed
Result: The operation produces the result #% but this is not stored
anywhere. The following instruction will branch to LOOP because the result of
the preceding operation was not equal to zero.

24. Homework Set #3 Solution
1. Write a subroutine to save the first 5 odd (8-bit) numbers pointed to by the x-register (passed in) onto the stack.
Note: For subroutine to execute in main program, a BSR or JSR command must be used in main program.
2. Write a program to output a square wave thru port S pin The output can be observed on the scope, and the period T of the wave should be measured. More than one period wave should be generated. The machine cycle time of the MC9S12C32 should be estimated. Draw the square wave.

25. QUESTIONS???