• Sets-Up the video screen for 24 x 40 text mode
• Transfers control to the ROM Cartridge, booted program, or
"blackboard" program
The remaining 2K of Operating System ROM is a floating point program
which is not used by the other parts of the Operating System itself,
but is available to non-resident programs such as BASIC, Calculator,
Pascal, etc. The floating point numbers are stored as ten BCD digits
of mantissa plus a one-byte exponent. The following routines are
among those found in the ROM program:
• ASCII to floating point and floating point to ASCII conversion
• Integer to floating point and floating point to integer conversion
• Floating point add, subtract, multiply, and divide
• Floating point log, exponent, and polynomial evaluation
• Floating point number clear, load, store and move
4.3.5 RAM Memory Boards
The ATARI 400 and 800 Computers come equipped with either 8K or 16K
RAM memory boards. The 8K and 16K RAM memory boards are the same
except for the 8K or 16K ememory chips and the addition of connector
jumpers on the 8K RAM memory boards. (See Figures 4-11, 4-12, 4-13,
and 4-14.) To address 8K of memory requires thirteen bits, and to
address 16K of memory requires fourteen bits. The 8K RAM Memory board
contains jumpers that are connected depending upon which RAM chip is
installed on the board. The jumper makes up one bit of the thirteen-
bit address. Figure 4-12 contains a jumper table detailing which
connection to make for each type of RAM memory chip.
4.3.6 Power Supply
The 400 and 800 Power Supply have identical circuitry, with a few
exceptions. (See Figures 4-15 through 4-18). The Console Power
Supplies receive 9Vac (J204 and J206, 400 and 800 respectively) from
the external power adapter (transformer) and provides +5Vdc, +12Vdc,
and -5Vdc for the Console. Both the 400 and 800 Power Supplies have
two power ON/OFF switches, S202 and S203 for the 400 Console, and
S201 and S202 for the 800 Console. S202 and S201, respectively, are
Power Interlock switches. Whenever the Program Cartridge door is
opened the Power Interlock switch removes power from the system.
4-16 System Service Manual
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