Commodore 128 Programmer's Reference Manual page 145

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^ MACHINE LANGUAGE ON THE COMMODORE 128 135
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1 You can see that the highest number of the high byte of the 16-bit address is
65280. And you know that the highest number of the low byte of the 16-bit address
equals 255. Add the highest high-byte and the highest low-byte number (65280 + 255),
to arrive at 65535, the highest address within each of the two 64K RAM banks.
When the microprocessor calculates the address of the next instruction, it looks at
p the high byte of the 16-bit program counter. Try to think of the high byte of the address
] ! as just another 8-bit byte. If this was the case, the bit positions would be labeled from 0
on the right through 7 on the left, just like the low byte of the address. Therefore, the
n largest number this 8-bit byte can represent again is 255 decimal.
{ | The value in the high byte determines which 256-byte block is accessed. These
256-byte blocks are referred to as pages. The high byte determines the page boundary of
the address, so the high byte is calculated in increments of 256 bytes. The high byte of
the program counter determines which of the possible 256 pages is being addressed. If
you multiply the number of possible pages, 255 by 256 bytes, you realize the highest
page starts at location 65280, decimal, the same number as in the high byte in Figure
5-6. Location 65280 is the highest page boundary addressable.
What if you want to address a memory location that does not lie on a page
boundary? That's where the low byte of the 16-bit address comes in.
The high byte of the program counter represents the 256-byte page boundary.
All addresses between boundaries are represented by the low byte. For example, to
address location 65380 decimal represent the high byte as 255, since 255 times
256 equals 65280. You still have to move 100 addresses higher in memory to location
65380.
The low byte contains the value 100 decimal. The low byte value is added to the
high byte to find the actual, or effective address.
When you look at the memory map of your Commodore 128, you will see
references to the low byte and high byte pointers or vectors to certain machine-language
routines within the operating system or to important system memory locations, like the
start of BASIC.
You can find out the contents of these addresses and where the routines reside in
your Commodore 128's memory by using the PEEK command in BASIC, or the
Memory command in the Machine Language Monitor. To find the effective address
using BASIC, look in the memory map for the reference to a specific routine or system
function, sometimes called a vector. PEEK the high byte, the page number of the
routine. Multiply by 256 to find the page boundary. Then PEEK the low byte and add it
to the page boundary to arrive at the effective decimal address.
Keep in mind that all the address calculations are performed in binary. They are
explained in decimal so they're easier to understand. In your machine language pro
grams, you will usually reference memory in hexadecimal notation, explained in the
next section.
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