- Recall:
- The M68000 CPU has
8 data registers, named:
- D0, D1, D2, D3, D4, D5, D6, D7, D8
- Each data register contains 32 bits
- The M68000 CPU has
8 data registers, named:
- Facts:
- Data registers are used
to store
the operands for
computer instructions
- There are many different types
of operands:
- byte typed that are 1 byte in size
- short typed that are 2 bytes in size
- int typed that are 4 bytes in size
- And so on...
- Data registers are used
to store
the operands for
computer instructions
- Use of data registers"
- Data registers
can provide source operands
of any data type
for use computer operations
- Data registers can be used as destination to store results of any data type produced by computer operations
- Data registers
can provide source operands
of any data type
for use computer operations
- Common practice: in
assembler programming:
- When you perform calculations
of variables
(that are stored in
memory),
you will
always:
- Make a
copy of the
variable in
some data register
- Then perform the
operations on the
copy in the
data register
- When the computations are done, you must store/save the result in the data register to the variable in memory !!!
Example:
- When you compute:
x = x + 1;The variable x is stored in memory
- You need to make a
copy of
x in
a data register
first
Then add 1 to obtain x + 1 in a data register
You must store the value x + 1 in the data register back to the variable in memory to complete the computation !!!
Warning:
- If you do not store the result back to the variable in memory, you will lose all the computational work that you have done !!!!
- Make a
copy of the
variable in
some data register
- When you perform calculations
of variables
(that are stored in
memory),
you will
always:
- Didactical comment:
- We will only use
(different) integer data types
to learn assembler programming
(Floating point data types follows a similar logic)
- We will only use
(different) integer data types
to learn assembler programming
- A data register of M68000
can store
3 different integer type
values:
- byte data type:
- A byte typed value in a data register is stored in the right most 8 bits of the data register
- short data type
- A short typed value in a data register is stored in the right most 16 bits of the data register
- int data type
- A int typed value in a data register is stored in the entire (= all 32 bits) of the data register
Graphically:
Example:
Before executing the MOVE instruction: +----------+----------+----------+----------+ D0: | 10101010 | 00001111 | 11110000 | 01010101 | +----------+----------+----------+----------+ +----------+----------+----------+----------+ D1: | 11111111 | 11111111 | 11111111 | 11111111 | +----------+----------+----------+----------+
After executing MOVE.B D0, D1: +----------+----------+----------+----------+ D0: | 10101010 | 00001111 | 11110000 | 01010101 | +----------+----------+----------+----------+ +----------+----------+----------+----------+ D1: | 11111111 | 11111111 | 11111111 | 01010101 | +----------+----------+----------+----------+ The byte operand in D0 is copied into the byte operand location of D1 !!!
After executing MOVE.W D0, D1: +----------+----------+----------+----------+ D0: | 10101010 | 00001111 | 11110000 | 01010101 | +----------+----------+----------+----------+ +----------+----------+----------+----------+ D1: | 11111111 | 11111111 | 11110000 | 01010101 | +----------+----------+----------+----------+ The word operand in D0 is copied into the word operand location of D1 !!!
After executing MOVE.L D0, D1: +----------+----------+----------+----------+ D0: | 10101010 | 00001111 | 11110000 | 01010101 | +----------+----------+----------+----------+ +----------+----------+----------+----------+ D1: | 10101010 | 00001111 | 11110000 | 01010101 | +----------+----------+----------+----------+ The long word operand in D0 is copied into the long word operand location of D1 !!! - byte data type:
- DEMO: click here