1. Chapter 3 Introduction to the 68000
Register Set: data, address, condition code, status.
Basic Instruction Set
Basic addressing modes: register, absolute, immediate, register indirect, etc.
Assembling and debugging a program

2. Register Set
8 general-purpose data registers. Word operation on D00-D15, byte operation on D00-D07
D31
D16
D08
D00 D0 D7 D6
PC points at the next instruction to be executed PC

3. Address Register: A0-A7 8 address registers of 32 bits.
Information in an address register represents a location in memory.
Special one: A7 is used as stack pointer.
Memory
1005
1004
1006
1007 4A 0E 07 57 A0
A31
A16
A00 A0 A1 A7
[M(A0)] = [M(1005)] = 57

4. Condition Code Register
Carry
oVerflow C V Z N X 7
Zero
Negative
eXtend
The CCR is updated to reflect the result of the operation.
Z=1 if the result is 0
C=1 if there is carry-out from MSB
V=1 if there is overflow
N=1 if the result is negative

5. Instruction Set The 68000 has a large instruction set.
Move data
Modify or operate the data
Change execution sequence
Determine the operation mode of CPU
e.g. MOVE.B D3,1234
MOVE.B #25,D2
Classification of instruction set architecture
CISC (Complex instruction set computer): large instruction set, powerful, but difficult to optimize code
RISC (Reduced instruction set computer): smaller instruction set, easy to optimize code, but longer program

6. Data Movement
Register-to-register, register-to-memory, memory-to-register, memory-to-memory, constant-to-memory/register.
8-bit, 16-bit, 32-bit correspond to MOVE.B, MOVE.W, MOVE.L
Legal: Assembler Form RTL
MOVE.B D1,D2 [D2]  [D1]
MOVE.B D3, 1234 [M(1234)]  [D3]
MOVE.B 1234,2000 [M(2000)]  [M(1234)]
MOVE.B #12,1234 [M(1234)]  12
Illegal: MOVE.B D3,#12 12  [D3] ???

7. Unconditional Branch BRA: Branch BRA address ; GOTO address Example:
BRA NEXT
MOVE.B D1,D2
NEXT MOVE.B #1,D4
Which instruction will be executed after BRA NEXT is executed?

8. Conditional Branch BEQ, BNE, BCC, BCS, BVS Example:
BCC Check_5 IF c=0 THEN branch to Check_5
MOVE.B D1,D2 …
Check_5 MOVE.B #1,D4
BCC Check_5
C=0
C=1
Check_5 MOVE.B #1,D4
MOVE.B D1,D2

9. Conditional Branches (con.)
BNE Branch on [CCR(Z)]=0
BEQ Branch on [CCR(Z)]=1
BCC Branch on [CCR(C)]=0
BCS Branch on [CCR(C)]=1
BVC Branch on [CCR(V)]=0
BVS Branch on [CCR(V)]=1
The complete set of conditional branch instructions is given in Table 5.2 on page 208. (e.g., BLT, BLE, BGT, BGE, etc.)

10. CMP and TST Useful for changing program flow CMP: Compare
Syntax: CMP src,Dn
Operation: [Dn] - [src]
Result of - is not saved
TST: Test an operand
Syntax: TST dest
Compare [dest] to 0, no result saved
TST D1 is the same as CMP __,D1

11. Change Program Flow
IF1 Set Flag Test Opposite Condition and BR to ENDIF if TRUE THEN1 IF-Part ENDIF1 ...
IF X1 = 0 THEN X1 := Y1
IF1 MOVE.B X1,D0 (or “TST.B X1”)
BNE ENDIF1
THEN1 MOVE.B Y2,X1
ENDIF1 … other code

12. IF X1 = 0 THEN X1 := Y1 ELSE X1 := Y2
Change Program Flow (con.)
IF X1 = 0 THEN X1 := Y1 ELSE X1 := Y2
Right
IF1 MOVE.B X1,D0
BEQ THEN1
ELSE1 MOVE.B Y2,X1
BRA ENDIF1
THEN1 MOVE.B Y1,X1
ENDIF1 ...
Wrong
IF1 MOVE.B X1,D0
BEQ THEN1
ELSE1 MOVE.B Y2,X1
THEN1 MOVE.B Y1,X1
ENDIF1 …

13. IF X1 = 0 THEN X1 := Y1 ELSE X1 := Y2
Change Program Flow (con.)
IF X1 = 0 THEN X1 := Y1 ELSE X1 := Y2
IF1 MOVE.B X1,D0
BNE ELSE1
THEN1 MOVE.B Y1,X1
BRA ENDIF1 must have this branch
ELSE1 MOVE.B Y2,X1
ENDIF1 ...

14. Change Program Flow (con.)
WHILE (K > 0) DO S
WHILE TST.B K
BLE ENDWH test opposite condition
S loop body
BRA WHILE
ENDWH ...

15. Change Program Flow (con.)
FOR I = N1 TO N2 DO S
MOVE.B N1,D0 D0: loop counter
NEXT CMP.B N2,D0
BGT ENDFOR
S loop body
ADD.B #1,D0
BRA NEXT
ENDFOR ...

16. 1 assembly language instruction = 1 machine language instruction
1 high-level language instruction  1 machine language instructions

17. Subroutine …
BSR ADD12
BSR ADD12
ADD12 ADD.B D1,D2
SUB.B #12,D2
RTS

18. Data Typing
Most high-level language, like Pascal, Java, Ada, are said to be strongly typed.
Assembly language is not strongly typed.
How about C/C++?
Example: A character can be multiplied by an integer.
JAVA?

19. Data Typing (con.) Memory Map A 1000 12 00 ORG $1000 A DC.B $12
B DC.W $3456
C DS.B 1
D DS.L 1
B 1002 34 56
C 1004 00 00
D 1006 00 00 00 00
MOVE.B A,D0
ADD.B B,D0 [D0]  $12 + $34
MOVE.B A,D0
ADD.W B,D0 [D0]  $12 + $3456
MOVE.W A,D0
ADD.W B,D0 [D0]  $ $3456
MOVE.L A,D0
ADD.L B,D0 [D0]  ?

20. Arithmetic Operation ADD, SUB, CLR, NEG, ASL, ASR
ADD.B 1234,D3 ; [D3]  [M(1234)]+[D3]
The CCR is updated accordingly.
Example: V3 = V1 + V2
signed integers unsigned integers
ORG $400
V1 DC.B 12
V2 DC.B 14
V3 DS.B 1
ORG $600
MOVE.B V1,D0
ADD.B V2,D0
MOVE.B D0,V3
BVS Error1
...
ORG $400
V1 DC.B 12
V2 DC.B 14
V3 DS.B 1
ORG $600
MOVE.B V1,D0
ADD.B V2,D0
MOVE.B D0,V3
BCS Error1
...
Data
Are the codes
correct?
Program
Where is the right place for BVS/BCS?

21. Arithmetic Operation (con.)
Subtraction: SUB src, dest ; [dest]  [dest] - [src]
SUB.B D2,D0 ; [D0(0:7)]  [D0(0:7)] - [D2(0:7)]
SUB.W D2,D0 ; [D0(0:15)]  [D0(0:15)] - [D2(0:15)]
SUB.L D2,D0 ; [D0]  [D0] - [D2]
Clear
CLR.B D ; [D0(0:7)]  0
Negation: negative value, i.e,, 2’s complement
NEG.B D4 ; 2’s complement of D4
If [D4] = , after [D4] =

22. ASL (Arithmetic Shift Left)
Operand
Format:
ASL #n,dest or
ASL Di,dest
shifts bits in dest LEFT by n or [Di] places, respectively
The bit shifted out is shifted in C-bit of CCR.
Example: ASL.B #3,D0
[D0] =
[C]=
[C]=
[C]=

23. ASL (Arithmetic Shift Left)
Why is ASL useful?
ASL is the fastest way to perform
“multiply by 2’s power”
ASL dest = ASL #1,dest
What does ASL #n, dest do?
[dest]  [dest] x 2n
How to multiply D0 by 2 ?
[D0] =
After ASL.B #1,D0
[D0] =
Can ASR cause overflow?

24. ASR (Arithmetic Shift Right)
Operand C
MSB
Same as ASL, but
bits shifted to RIGHT
MSB is duplicated back into MSB (Why?)
ASR.B #1,D0 is equivalent to dividing D0 by 2
How to divide
D1 by 32 ?
Example: [D0] = -22 =
After “ASR.B #1,D0”
[D0] = = -11
[CCR(c)] = 0
Can ASR cause overflow?

25. Effect of Arithmetic Operations on CCR
Addition:
 1, if carry out from MSB
C = 
 0, otherwise
 1, if operands are of same sign and
V =  their sum is of the opposite sign
____ ____ ____
V= an-1  bn-1  sn-1 + an-1  bn-1  sn-1
where an-1, bn-1, sn-1 are the MSBs of
source destination and result, respectively

26. Effect of Arithmetic Operations on CCR
Subtraction:
 1, if NO carry out from MSB
C = 
 0, otherwise
 1, if operands are of opposite sign and
V =  the result is of same sign as the source
____________
V= (an-1  bn-1)  (dn-1  an-1)
where an-1, bn-1, dn-1 are the MSBs of
source destination and result, respectively

27. Logical Operation AND, OR, EOR, NOT If [D0] = 11110000
AND.B #% ,D0 ;[D0]=
OR.B #% ,D0 ;[D0]=
EOR.B #% ,D0 ;[D0]=

28. Use Registers
Accesses to data registers are faster than accesses to memory.
Shorter instruction, 3 bits to indicate either one of 8 general-purpose registers.
Use comments to indicate how registers are used in program.
* GetChar: Input an ASCII-coded character into D0
* Input Parameters: None
* Output parameters: ASCII character in D0, Error code in D6
* Registers modified: D0, D1, D6
GetChar MOVE.B ACIAC,D1
BTST.B #RDRF,D1
BEQ GetChar
MOVE.B ACIAC,D0
AND.B #% ,D1
MOVE.B D1,D6
RTS

29. Addressing Modes
Concerned with the way in which data is accessed (where operand can be found)
Data register direct, absolute, immediate, and address register indirect.
Looking for a house in a familiar neighborhood, “The house next to Tim’s” (relative location) is enough.
Looking for a house in a new environment, “61 William Street” (actual address) is necessary, even with a city name.

30. Absolute Addressing
Use the actual or absolute address of the operand, e.g. CLR.B $234
MOVE.B D2, $2000
The source is data register direct, a type of absolute addressing mode. $2000 is memory location 2000
Symbols can also be used
Example:
MOVE.B Input,D0
SUB.B Time,D0

31. Immediate Addressing MOVE.B #25,D2 ; [D2]  25
Immediate addressing is faster than the absolute addressing.
Data 25 is part of the instruction  stored in IR.
Absolute Addressing:
Hours DC.B 25
ADD.B Hours,D2
Immediate Addressing:
Hours EQU 25
ADD.B #Hours,D2
What about “ADD.B #Hours,D2
What about “ADD.B Hours,D2

32. IF 7<P<25 THEN X := 6 if ((7<P) && (P<25)) X = 6;
MOVE.B P,D0
CMP.B #7,D0
BLE OutOfRange
CMP.B #25,D0
BGE OutOfRange
MOVE.B #6,X
OutOfRange …
IF 7<P<25 THEN X := 6 if ((7<P) && (P<25)) X = 6;
CLR.B D1 D1: X
MOVE.B
#1,D0 D0: I
NEXT ADD.B D0,D1
ADD.B #1,D0
CMP.B #10,D0
BLE NEXT
MOVE.B D1,X
X := 0 FOR I := 1 TO X = X + I X = 0; for (I = 1; I <= 10; I++) X = X + I;

33. Address Register Indirect
The address of an operand is found in an address register, A0 to A7
Pointer or reference
MOVEA: copy an address to address reg
MOVEA.L #$1000,A0
CLR.B (A0)
same effect as
CLR.B $1000
1000 A0
0FFF
1001
1002
1003 00

34. Indirect Addressing Two accesses:
1) to the address register A0 to find the actual address of operand, 1000.
2) to the memory location 1000 to get the operand.
Why is address register indirect addressing useful?

35. Example: Add 100 numbers together, the data starts at memory location 200016
MOVE.B $2000,D0
ADD.B $2001,D0
ADD.B $2002,D0
ADD.B $2003,D0 :
ADD.B $2063,D0
CLR.B D0
MOVEA.L #$2000,A0
NEXT ADD.B (A0),D0
ADDA.L #1,A0
CMPA.L #$2064,A0
BNE NEXT

36. Different Addressing
* Program to test the different addressing modes.
* [D2]  [M(0)] By Mingrui Zhang 3
ORG $1000
A ABSOL: DC.B 26
A IMMED: EQU 26 7
ORG $2000
C MOVEA.L #$1000,A0
MOVE.L $0,D2
A D ADD.B (A0),D2 12
C MOVE.L $0,D2
D ADD.B ABSOL,D2 15
MOVE.L $0,D2
A ADD.B #IMMED,D2 18
C 4E STOP #$2700
END $2000

37. The Teesside MC68000 Cross-assembler and Simulator
TOOLS FILES
text editor Test.X68
X68K Test.BIN (and Test.LIS)
E68K
Desing the program
Edit source file
Cross-assemble the program
Run the program

38. Debugging Commands HELP: Provide information about commands
MD (.): Displays the contents of memory
e.g. MD 400
MD 400 -DI ;disassemble the contents of
memory.
MM: Memory modification
e.g. MM 400 -B MM W
MM 400 -B -DEC MM W -DEC
DF: Displays the contents of all registers
.PC: set PC
e.g. .PC 400
GO: Execute program, (ESC to escape)
TR: Executes a single instruction at a time
BR 10000: Places a marker at location 10000
QU: Quit