Commodore VIC-20 User Manual page 291

Chapter 8 Peripheral Devices
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by the system, you may use that memory freely. One method is to specify
which buffers you want when you open a disk channel and write your
routines in one or two of the buffers that are not in use.
The information that follows is not intended for beginning pro-
grammers. The use of the MEMORY-READ, MEMORY-WRITE, and
MEMORY-EXECUTE commands requires a thorough understanding of
machine language programming and the Disk Operating System.
MEMORY-WRITE
To store data into the disk drive memory, you will need to use the
MEMORY-WRITE command. Like POKE in BASIC, this instruction
puts whatever data you specify into any memory location you want. Take
the example of a POKE statement.
POKE
768J 255
There is only one byte transferred with each POKE statement. The
MEMORY-WRITE command allows you to transfer up to 34 bytes with a
single statement. To perform the same operation using MEMORY-WRITE
you would need to convert the decimal memory location into a hexadecimal
number; 300 is the hexadecimal equivalent of decimal 768. Since you can
transfer only one byte at a time with a POKE command, BASIC always
knows how many bytes to expect. With the MEM OR Y -WRITE command,
you will need to indicate how many bytes are going to be transferred. In this
case, only one byte is going to be transferred, so the command will be quite
short.
There are some special constraints that must be used with these instruc-
tions since they are actually just extensions of machine code.
1. The memory address must be entered as two bytes, low byte first,
then high byte.
2. All of the data must be transferred as character strings (CHR$).
3. The 6502 understands only binary data. The instruction allows you
to enter numbers in hexadecimal; but BASIC doesn't use that
notation. For instance,
65536 decimal = FFFF hexadecimal
This is two hexadecimal bytes (FF and FF). To represent them
in this mode, take the two bytes and convert each to its decimal