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Machine Language Tutorial Disk Part 2
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* *
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* MACHINE LANGUAGE TUTORIAL DISK *
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* WRITTEN BY DR. FIRMWARE *
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PART II
Machine language command structure.
Even though this sounds complicated,
the structure of machine language
commands is quite simple. The command
is one to three bytes long and consists
of two sections, the operator and the
arguement. The operator is always one
byte long and the arguement is either
zero, one or two bytes long. If the
arguement is zero bytes long, then
it is said that there is no arguement
for that command.
The accumulator
The accumulator is the primary register
in the 6502 microprocessor. It is an 8
bit register, which means that it can
handle only eight bits at a time or the
numbers from zero to 255.
To put numbers into the accumulator,
we use a command called LDA which
stands for LoaD Accumulator. This
command takes the value generated by
the arguemant and places it into the
accumulator.
Addressing modes
Addressing modes are very important.
These tell the computer how to deal
with the arguement that it recieves. We
will only be dealing with two modes for
the present, immeadiate, and absolute.
In immeadiate addressing mode, the LDA
command load the accummulator with the
actual value of the arguement. Suppose
that we wanted to load the value $6F
into the accumulator. We would do this
by telling the microprocessor to
'LDA #$6F'. That is assembly language.
In actual fact, the code used by the
microprocessor would represent it as
'$A9 $6F'. The $A9 tells the
microprocessor that you want to load
the accumulator in immeadiate
addressing mode. The $6F is the
arguement and is treated as described
above. So then, the number $6F is put
directly into the accumulator.
The LDA command in immeadiate
addressing mode is two bytes long. The
first byte being the operator ($A9) and
the second being the arguement.
Memory locations.
The Apple computer has 2~16 memory
locations. Each memory location is 8
bits large. Each memory location can be
referenced by a 4 digit hex number.
A four digit hex number is 2 bytes long
and can be cut in half into two
separate bytes. The byte on the left is
more significant than the one on the
right, so the one on the left is called
the Most Significant Byte (MSB) and the
one on the right is the Least
Significant Byte (LSB).
In absolute addressing mode, the LDA
command takes the arguement as an
address and then takes the value held
in that address and transfers it to the
accumulator. The arguement is two bytes
long and it forms the address LSB first
and MSB second. The address is in
effect backwards.
Say you wanted to load the accumulator
with whatever was in location $456D.
The operator is $AD, this is followed
by the LSB which is $6D, and finally
the MSB, $45.
Storing the accumulator.
To move the contents of the accumulator
to some other memory location, we use
the command STA, which stands for STore
Accumulator.
The STA command has an absolute
addressing mode. The hex operator is
$8D and it is followed by the LSB and
MSB, in that order. After the command
is executed, the accummulator still
contains the value.
Now we can make a tiny program to store
the value $8D into location $2000.
First, we have to load it into the
accumulator. To do this, we'll load the
$8D into the accumulator through the
LDA immeadiate command. So, then we'll
store the accumulator into $2000 while
it contains our value using the STA
absolute command.
In assembly language, our program looks
like this:
LDA #$8D
STA $2000
RTS
Note: the '#' indicates that the
command is in immediate addressing
mode. The RTS is going to be used as a
general 'end' command for now, until I
can explain it's actual usage.
This assembly language version is not
understandable by the microprocessor.
It has to be translated into hex codes.
This translation is normally done by an
assenbly program, but since this is a
short program, we'll do it by hand.
We are going to put this program at
location $300-$306. This area can be
used for short programs as $300-$3b0 is
free memory space. An extended memory
map will be included in a later
edition.
LDA #$8d --> $A9 8D
STA $2000 --> $8D 00 20
RTS --> $60
hex location contents
---------------------------
$300 $A9
$301 $8D
$302 $8D
$303 $00
$304 $20
$305 $60
The program can be entered into memory
using the BASIC POKE command. $300 is
equal to 768 and the rest of the hex
numbers you should be able to convert
into decimal yourselves.
This concludes PART II of the series.
Coming next: X and Y registers.
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DR. FIRMWARE, 1985.
I CAN BE REACHED ON TESTY, 514-332-6852
OR ON TRANSFERS AE, 514-738-1247
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