Atari* System Reference manual
(c) 1987 By Bob DuHamel
6915 Casselberry Way
San Diego, CA 92119
This manual contains highly technical information. Such information
is provided for those who know how to use it. To understand the
advanced information you are expected to know 6502 assembly language.
If you are new to programming, concentrate on the parts which discuss
Addresses are usually given in both hexadecimal and decimal numbers.
The operating system equate names are given in capital letters with
the address following in brackets. The decimal address is in
parenthsis within the brackets. For example:
DOSVEC [$000A,2 (10)]
name hex dec
The ",2" after the hexadecimal number means that this address requires
two bytes to hold its' information. Any address called a "vector"
uses two bytes whether noted or not.
Control registers and some other bytes of memory are shown in the
7 6 5 4 3 2 1 0
1 6 3 1 8 4 2 1
2 4 2 6
The numbers on top are the bit numbers. Bit 7 is the Most Significant
Bit (MSB) and bit 0 is the Least Significant bit (LSB). The numbers
on the bottom are the bit weights. These are useful when changing
memory with decimal numbers, as you would in BASIC. For example, to
set bit 4 of a register to 1, without changing any other bits you
would add 16 to the decimal number already in the register. To reset
the same bit to 0, you would subtract 16 from the number in the
register. This is exactly what the command GRAPHICS 8+16 does. It
sets bits 3 and 4 of a graphics mode control register.
MSB and LSB may also mean Most Significant Byte or Least Significant
Byte, depending on context.