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68000 Architecture, Data Types and Addressing Modes

68000 Architecture, Data Types and Addressing Modes. Lecture Overview. Assembler Language Programmers Model Registers Addressing Modes. Assembler Language. Programming language very close to the machine code for a language Can be considered a native language for the architecture targeted.

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68000 Architecture, Data Types and Addressing Modes

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  1. 68000 Architecture, Data Types and Addressing Modes Lecture 2 - Prog Model

  2. Lecture Overview • Assembler Language • Programmers Model • Registers • Addressing Modes Lecture 2 - Prog Model

  3. 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 Lecture 2 - Prog Model

  4. 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 Lecture 2 - Prog Model

  5. 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 Lecture 2 - Prog Model

  6. 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 Lecture 2 - Prog Model

  7. Assembler Code Example Lecture 2 - Prog Model

  8. 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 Lecture 2 - Prog Model

  9. 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 Lecture 2 - Prog Model

  10. 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 Lecture 2 - Prog Model

  11. 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 Lecture 2 - Prog Model

  12. Status Word Lecture 2 - Prog Model

  13. 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 Lecture 2 - Prog Model

  14. 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 Lecture 2 - Prog Model

  15. 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)] Lecture 2 - Prog Model

  16. 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 2nd register listed is both the 2nd operand of the operation and gets the results of the operation. • Example: MOVE.L D0,D3 • Effect: [D3]  [D0] Lecture 2 - Prog Model

  17. Address Register Indirect • Register Indirect means that the address of the operand is in a register, the address registers. • Ex: Lecture 2 - Prog Model

  18. 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? Lecture 2 - Prog Model

  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 Lecture 2 - Prog Model

  20. 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 Lecture 2 - Prog Model

  21. 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? Lecture 2 - Prog Model

  22. 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. Lecture 2 - Prog Model

  23. 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. Lecture 2 - Prog Model

  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 Lecture 2 - Prog Model

  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. Lecture 2 - Prog Model

  26. You can get a free 68K assembler/simulator Lecture 2 - Prog Model

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