Memory in the system is decoded in the full 64K range of the 6502 microcomputer and there are no provisions for additional mapping to extend memory. Memory is divided into four basic regions (with some overlap possible): RAM, cartridge area, I/O region and the resident OS ROM. The regions and their address boundaries are listed below (all addresses are in hexadecimal):
Figure 4-1 6502 System Memory Map:
0000-1FFF = RAM (minimum required for operation) 2000-7FFF = RAM expansion area 8000-9FFF = Cartridge B, Cartridge A (half of 16K size) or RAM A000-BFFF = Cartridge A or RAM C000-CFFF = Unused D000-D7FF = Hardware I/O decodes D800-DFFF = Floating Point Package (OS) E000-FFFF = Resident Operating System ROM
This section will break these regions into even smaller functional divisions and provide detailed explanations of their usage.
RAM REGION
The OS and the control program share the RAM region. The RAM region can be further subdivided into the following sub regions for discussion purposes:
Page 0 = 6502 page zero address mode region. Page 1 = 6502 stack region. Pages 2-4 = OS database and user workspace. Pages 5-6 = User program workspace. Pages 7-XX = Bootable software area/free RAM.* Pages XX-top of RAM = Screen display list and data.*
* Note that XX is a function of the screen graphics mode and the amount of RAM installed.
The paragraphs that follow describe how the OS uses RAM subregions, and presents user program recommendations.
Page 0
The architecture of the 6502 microcomputer instruction set and addressing modes gives page 0 special significance. References to addresses in that page (0000 to 00FF) are faster, require fewer instruction bytes, and provide the only mechanism for hardware indirect addressing. Page 0 should be used sparingly so that all possible users can have a portion of it. The OS permanently takes the lower half of page 0 (0000 to 007F). This portion can never be used by any outer environment unless the OS is completely disabled and all interrupts to the OS are eliminated.
The upper half of page 0 (0080 to 00FF) is available to outer environments with the following restriction: the floating-point package, if used, requires 00D4 through 00FF.
Page 1
Page 1 is the 6502 hardware stack region; JSR instructions, PHA instructions, and interrupts all cause data bytes to be written to page 1. Conversely RTS, PLA, and RTI instructions all cause data bytes to be read from page 1. The 256-byte stack is adequate for normal subroutine calls plus interrupt process nesting. so no restrictions have been made on page 1 usage It is obvious that a stack of this size is totally inadequate for deeply recursive processes or for nested processes with large local environments to be saved. So, for sophisticated applications, software maintained stacks must be implemented.
The 6502 stack pointer is initialized at power-up or system reset to point to location 01FF. The stack than pushes downward toward 0100. The stack will wrap around from 0100 to 01FF if a stack overflow condition occurs, because of the nature of the 6502's 8-bit stack pointer register.
OS Data Base
Locations 0200 thxough 047F are allocated by the OS for working variables, tables and data buffers. Portions of this region can be used only after you determine that nonconflict with the OS is guaranteed. For example, the printer and cassette buffers could be used if I/O operations to these devices are impossible within the controlling environment. The amount of work involved in determining nonconflict seems to be completely out of line with the benefits to be gained (except for a few trivial cases) and it is recommended that pages 2 through 4 not be used except by the OS.
User Workspace
Locations 0480 through 06FF are dedicated for outer environment use except when the floating paint package is used. The floating-point package uses locations 057E through 05FF.
Boot Region
Page 7 is the start of the "boot region." When software is booted from either the diskette or the cassette, it can start at the lowest free memory address (that is 0700) and proceed upward (although it can also start at any address above 0700 and below the screen display list). The top of this region defines the start of the “free memory" region. When the boot process is complete, a pointer in the data base contains the address of the next available location above the software just booted. When no software has been booted, this pointer contains the value 0700.
Screen Display List and Data
When the OS is handling the screen displays the display list that defines the screen characteristics and the current data that are contained on the screen are placed at the high address and of RAM. The bottom of this region defines the end at the free memory region and its location is a function of the screen mode currently in effect. A pointer in the data base contains the address of the last available location below the screen region.
Free Memory Region
The free memory region is all the RAM between the end of the boot region and the start of the screen region. The outer level application is responsible for managing the free memory region.
CARTRIDGES A AND B
There are two 8K regions reserved for plug-in cartridges. Cartridge B, that is the right-hand cartridge slot found only in the ATARI 800 Home Computer, has been allocated memory addresses 8000 through 9FFF. Cartridge A (the left-hand cartridge slot in the ATARI 800 Computer console, and the only slot in the ATARI 400 Computer console) has been allocated memory addresses A000 through BFFF and optionally 8000 through BFFF, for 16K cartridges. If a RAM module is plugged into the last slot such as to overlay any at these addresses, the RAM takes precedence as long as a cartridge is not inserted. Howevers if a cartridge is inserted, it will disable the entire conflicting RAM module in the last slot in 8K increments.
