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

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

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 9/20/6 Lecture 2 - Prog Model

Assembler Directives TTL EQU “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 9/20/6 Lecture 2 - Prog Model

Assembler Directives (2) 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 DC.L $0A1234 Store $000A1234 in memory STR DC.B ‘April’ Store ASCII values 9/20/6 Lecture 2 - Prog Model

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 9/20/6 Lecture 2 - Prog Model

Assembler Code Example 9/20/6 Lecture 2 - Prog Model

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 9/20/6 Lecture 2 - Prog Model

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 9/20/6 Lecture 2 - Prog Model

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 9/20/6 Lecture 2 - Prog Model

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 9/20/6 Lecture 2 - Prog Model

Status Word 9/20/6 Lecture 2 - Prog Model

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 9/20/6 Lecture 2 - Prog Model

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 9/20/6 Lecture 2 - Prog Model

9/20/6 Lecture 2 - Prog Model

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)] 9/20/6 Lecture 2 - Prog Model

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] 9/20/6 Lecture 2 - Prog Model

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

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? 9/20/6 Lecture 2 - Prog Model

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 9/20/6 Lecture 2 - Prog Model

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 9/20/6 Lecture 2 - Prog Model

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? 9/20/6 Lecture 2 - Prog Model

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. 9/20/6 Lecture 2 - Prog Model

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. 9/20/6 Lecture 2 - Prog Model

Stack Operations PUSH data onto stack POP data from 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 9/20/6 Lecture 2 - Prog Model

Assignment From Text HW1 Problem 2-1 parts a,b,d,g Problem 2-5 9/20/6 Lecture 2 - Prog Model