68332 Basic Instructions

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68332 Basic Instructions

• Structure of an instruction

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Basic Instructions

• Each instruction has an associated instruction
  template.
• This defines the word data required to encode the
  instruction (i.e.just fill in the bits).

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• Source Where to find data
• Destination Where to copy data
• Size field
• 01 -gt byte operation
• 11 -gt word operation
• 10 -gt long operation
• Register Which register to use (if none then
  used for other purposes. )
• Mode Defines the data format/location

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Copying from Data Register to Data Register
Destination Mode 000 Source Mode
000 Example move a word of data from d5 to
d3
The instruction is fully self contained -gt no
extra operand data is necessary!
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Example Program

• Copy value in register d3 to register d0, d1 and
  d2
• What instruction sequence is required?
• Move word d3 -gt d0
• Move word d3 -gt d1
• Move word d3 -gt d2
• Where do we locate the code in memory? -gt 4000

4000 30 03 D0 lt- D3 4002 32 03 D1 lt-
D3 4004 34 03 D2 lt- D3
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Copying from Memory to Data Register
-gt Source Memory -gt Source Mode
111 Source register field encodes memory
address length 000 if memory address is
16-bit 001 if memory address is 32-bit Assume
all addresses are 32-bit (long-words). 16-bit
address used for compact programs and limit the
available address space.
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Example
Copy (word) contents of memory at 2000 into
register D3
This instruction requires an operand memory
address
4000 36 39 Move 2000 -gtD3 4002 00 00
4004 20 00
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Why is the operand 2 words in length?

• We are not moving the value 2000 into d3
  register, rather the contents of memory location
  2000
• Note
• We may not know the contents at this point.
• This is known as absolute addressing

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Example Program
4000 3639 Move 2000 -gt D3 4002 0000 4004 20
00 4006 3003 D0 lt- D3 4008 3203 D1 lt-
D3 400A 3403 D2 lt- D3
This program copies the word at 2000 into D3 and
then copies from there into D0, D1, and D2
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How is the PC updated?

• Recall the FETCH-gtDECODE-gtEXECUTE cycle
• CPU determines the number of operands by looking
  at the mode fields of the instruction during
  decoding
• -gt It adds 4 to the PC to skip over the 4 byte
  operand
• Remember the PC is immediately incremented by 2
  as soon as the instruction is read in

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The Complete Sequence of events
1. Fetch instruction at 4000 2. PC lt- PC
2 3. Decode instruction 4. PC lt- PC
4 5. Execute Move word from 2000 -gt D3 6. Fetch
instruction at 4006 7. PC lt- PC 2 8. Decode
instruction 9. Execute Move data from D3 -gt
D0 10. Fetch instruction at 4008 11. PC lt- PC
2 12. Decode instruction 13. Execute Move
data from D3 -gt D1 14. Fetch instruction at
400A 15. PC lt- PC 2 16. Decode
instruction 17. Execute Move data from D3 -gt D2
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Copying from Memory to Memory
-gt Source Destination Memory -gt Source
Destination Mode Register 111 000 If
memory address is 16-bit 111 001 If memory
address is 32-bit Example Move a word from
2002 to 200A 0 0 11 001 111
111 001 -gt 33F9
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Memory to Memory Program
4000 33F9 move.w 2002 -gt 200A 4002 0000
Source operand 4004 2002 4006 0000
Destination operand 4008 200A

• This instruction requires 2 operands, each 2
  words long.
• The source always precedes the destination if
  multiple operands are required.
• The PC will have to be incremented by 8 Bytes (as
  well as the normal 2 bytes) to point to the next
  instruction.

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Immediate Value to Data Register

• In previous examples we have referred to either a
  memory location or a data register.
• -gt A location where the data is stored.
• Sometimes we need to specify a given value known
  as an immediate value.
• -gt Use immediate addressing mode.

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Example Move Value 10 into D2
-gt Source mode immediate mode
111 -gt Source register immediate mode
100 0 0 11 010 000 111
100 -gt 343C
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Immediate Values Symbol

• We distinguish immediate values from addresses
  using sysmbol
• 1000 Decimal value 1000
• 1000 Hexadecimal value 1000
• 1000 Address 1000 (decimal)
• 1000 Address 1000 (hexadecimal)

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Mnemonics
Template for a move instruction.

• Difficult to remember templates for each
  instruction.
• Well replace this with the sysmbol (mnemonic)
• move
• Replace the size feld with the b, w and l
  subscripts
• move.b -gt move a byte
• move.w -gt move a word
• move.l -gt move a long word

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Encode Operands and the Destination/Source Modes

• The operands and the destination /source modes
  are encoded as arguments following the
  symbol(src, dest)
• move.b d0,d1
• move.w 2000,d5
• move.l 2000, 200A
• move.l 10, d2
• Assembler sysntax for the move command is
• move lteagt lteagt
• lteagt effective address an expression which,
  when evaluated, produces a value.

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Move
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Defaults

• In order to simplify coding of assembly language,
  certain defaults are assumed
• move -gt move.w (i.e. word size is default)
• move 10,d2 -gt absolute addressing
• 10 -gt decimal 10

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Addressing Restrictions
Only Bytes may be read from ODD memory
addresses. Words and Long-words must be read
from EVEN memory addresses!

• This was a design decision to simplify memory
  interfacing.
• Accessing words or long-words on odd boundaries
  causes the 68000 to terminate the F-gtD-gtE cycle.
• -gt An address error occurs

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Which of these instruction is legal?
a) move.w 4001, d0 b) move.b 4004, d1 c) move.b
4003, d3 d) move.w 4001, 4003 e) move.b 4000,
4001