1. 9/20/6Lecture 2 - Prog Model1 68000 Architecture, Data Types and Addressing Modes

2. 9/20/6Lecture 2 - Prog Model2 Lecture Overview  Assembler Language  Programmers Model Registers  Addressing Modes

3. 9/20/6Lecture 2 - Prog Model3 Assembler Language  Programming language very close to the machine code for a language Can be considered a native language for the architecture targeted. Assembler languages have a 1-to-1 translation to machine code  All assemblers have two types of statements Executable instructions Assembler directives

4. 9/20/6Lecture 2 - Prog Model4 Assembler Directives  TTL “Title” – gives the program a user-defined name  EQU **Links a pneumonic name to a value. Very valuable in making the program “readable”. Example to reserve 128 bytes for a stack. STACK_FRAME EQU 128 Define a stack frame of 128 bytes *** LINK A1,# -STACK_FRAME

5. 9/20/6Lecture 2 - Prog Model5 Assembler Directives (2)  ORG – sets the location counter (address)  DC Define constant. Allows the programmer to set memory locations to specific values. Qualifier specifies data type:.B,.W,.L ORG $001000 FST DC.B 10, 66 Set constants of 10 and 66 at memory location $1000 DC.L $0A1234 Store $000A1234 in memory STR DC.B ‘April’ Store ASCII values

6. 9/20/6Lecture 2 - Prog Model6 Assembler Directives (3)  DS Reserve bytes, words, or longwords of storage  ORG Define the value of the location counter. Sets the memory address where the code is placed.  SECTION Set the address counter to zero for position independent code.  END

7. 9/20/6Lecture 2 - Prog Model7 Assembler Code Example

8. 9/20/6Lecture 2 - Prog Model8 Programmers Model  When looking at any architecture what does the programmer see? REGISTERS (General purpose) ADDRESS REGISTERS SPECIAL PURPOSE REGISTERS MEMORY Allowable operations and data transfers supported

9. 9/20/6Lecture 2 - Prog Model9 DATA REGISTERS  A 32-bit machine  8 general purpose  Called D0 to D7  Any operation permitted on Di can also be done on Dj  Data access 8,16,32 bits

10. 9/20/6Lecture 2 - Prog Model10 Address Registers  8 Address Regs  Labeled A0 to A7  Entire 32 bits is single entity value  Any operation permitted on Ai is also allowed on Aj  2’s complement value treatment  Used in instructions to address memory

11. 9/20/6Lecture 2 - Prog Model11 Special Purpose Registers  2 such registers PC – Program Counter  Contains the address of the next instruction to be executed. SR – Status Register  16 bits divided into two bytes  MSB – System byte – control operation mode  LSB – Condition codes

12. 9/20/6Lecture 2 - Prog Model12 Status Word

13. 9/20/6Lecture 2 - Prog Model13 Addressing Modes  Modes supported are reflective of even the most modern architecture  Some Notation [Dx] means the “contents of” % num means binary $ num means hexadecimal  indicates transfer of information  Table on page 27 lists meaning of symbols and notation

14. 9/20/6Lecture 2 - Prog Model14 Immediate Addressing  ACTION: The value of the operand (data) is loaded into the specified register at the time the program runs.  The value of the operand is used in the operation  Note that general form of instructions is Operation #Source, Destination MOVE.W #$8123,D3 [D3(0:15)]  $8123  Fill the low order bytes of De MOVE.L #$8123,D3 [D3(0:31)]  $00008123  Fills all 32 bits of register D3

15. 9/20/6Lecture 2 - Prog Model15 Absolute Addressing  Sometimes called “Direct Addressing”  The address in memory being referenced by the instruction is given in the instruction. Used when the address of the operand is known when the program is written.  The instruction contains the operand’s address FORM Operation source,destination MOVE.L D3,$1234 [M($1234)]  [D3(16:31)] [M($1236)]  [D3(0:15)] MOVE.W $1234,D3 [D3(0:15)]  [M($1234)]

16. 9/20/6Lecture 2 - Prog Model16 Register Direct Addressing  Register to register operation  The assembler instruction lists both register which will be used for operand of the operation to be performed  The 2 nd register listed is both the 2 nd operand of the operation and gets the results of the operation.  Example: MOVE.L D0,D3 Effect: [D3]  [D0]

17. 9/20/6Lecture 2 - Prog Model17 Address Register Indirect  Register Indirect means that the address of the operand is in a register, the address registers.  Ex:

18. 9/20/6Lecture 2 - Prog Model18 Forms of Address Reg. Indirect  Address Register Indirect with postincrement The contents of the address register are incremented by the appropriate amount after use That is, the data is referenced with the value currently in the address register. Then the contents of the register are incremented by the size referenced.  Address Register Indirect with predecrement Contents are decremented and then used What use may these have?

19. Forms of Address Reg. Indirect (2)  Address Register Indirect with displacement A 16-bit displacement is specified in instruction Displacement added to address register to obtain the effective address of the operand  Register Indirect with Index Addressing Effective address is calculated by adding the address register + 8-bit signed displacement + contents of general purpose register Ref fig 2.15 of text, pg.39 9/20/6Lecture 2 - Prog Model19

20. 9/20/6Lecture 2 - Prog Model20 PC Relative  Similar to register indirect except that the address of the operand is specified with respect to the PC contents.  Allows data to be located relative to the location of the program in memory, i.e., relative to the Program Counter – allows WHAT????  Restricted to only the source operand

21. 9/20/6Lecture 2 - Prog Model21 PC Relative  Allows data to be located relative to the location of the program in memory, i.e., relative to the Program Counter – allows position-independent code.  What is position-independent code?

22. 9/20/6Lecture 2 - Prog Model22 PC Relative  Allows data to be located relative to the location of the program in memory, i.e., relative to the Program Counter – allows position-independent code.  What is position-independent code? Code that can be located anywhere (some caveats) in memory and executes the same.

23. 9/20/6Lecture 2 - Prog Model23 The Stack  With autoincrementing/autodecrementing of address register you have essentially 8 stacks. In this way register A0 through A7 can become user defined stacks  For Jump to Subroutine, register A7 is used So you may not want to use address register A7 for programs that calls to subroutines  The stack grows down and the register points to the TOS element and has valid contents.

24. Stack Operations  PUSH data onto stack Stack grows down. TOS has valid data. Use the predecrement form  MOVE.W $4234, -(A7)  POP data from stack Use the postincrement form  MOVE.W (A7)+,D4 9/20/6Lecture 2 - Prog Model24

25. Assignment  From Text HW1 Problem 2-1 parts a,b,d,g Problem 2-5 Problem 2-6 BE READY TO DISCUSS these in class next week. 9/20/6Lecture 2 - Prog Model25

26.  You can get a free 68K assembler/simulator 9/20/6Lecture 2 - Prog Model26