Freezer Upgrade

There is one upgrade that can be applied to an Atari XL/XE which is near and dear to my heart.... extra memory (256K). Many programs that are only OK when run from a disk drive, come alive if you execute them from memory. The PaperClip spelling checker is a good example of this. If you have a 256K machine, the dictionary will load completely into memory and will search a list of words instantly. There are also programs whose capacity is increased tremendously by the 256K upgrade. (PaperClip, a word processor, will hold 112,000 characters vs. AtariWriter's 20,000 or so). Great stuff, but..... a few complaints. How do you use a ramdisk with a program or DOS that is not written for them?? Also, when you turn off the power to a 256K memory chip, the data that is stored in it does not disappear in a few hundredths of a second as it did with the older and less efficient 16K and
64K devices. The operating system, which controls the power on and reset sequences, only checks a few bytes of memory to determine if the power was just turned on (the locations would be garbage), or if you had hit the RESET key (the locations would equal specific values). If, after powering off and then on (in order to re-boot your system), those bytes have retained their data, the system may branch through a warm (RESET key) start, instead of taking the proper path through cold (power on) start. This forces you to allow enough time to elapse after power down for those key addresses to lose their data. A repeat of power off/power on isn't going to help unless you wait the required interval (like 10 to 15 seconds...). This waiting around is very annoying - didn't you get this extra ram to save time?? So?? This is nothing new to those of you that have expanded systems. How do we fix it?? Relief arrives as a small hardware modification that allows you to force a cold start and boot the system with the RESET key (which will normally produce a warm start and no boot). With this circuit installed, you can re-boot your computer without turning off the power and losing the data in the extended memory banks. This means that you can install a ramdisk, load it with data and then re-boot the system from the ramdisk. Using a menu created for this purpose, programs that do not support a ramdisk - even a game, can be run - if you can force a coldstart without turning off the power (anyone want to write the menu for this??) Also, you could be operating with a ramdisk, boot a different program, run for a while, and then re-boot the original program with the ramdisk memory intact. And, of course, you don't have to wait for the memory to blank out after you power off. (a warm start is cooled down to a cold start any time you wish..... hence, the FREEZER.) This is accomplished by making the computer think that you have changed
the status of the cartridge, either removed one that you were using, or plugged one in where you had not had one before. During reset, the operating system checks the cartridge status since the last power on. If it sees that the status
has changed, it executes a cold start and re-boots the computer. This upgrade allows you to change that status when you press the FREEZER switch (the one you will add). This means that if you hold the FREEZER switch down, push RESET (and OPTION, if you don't want BASIC), and wait until the screen goes black(off), you will get a cold start. If you hold the FREEZER switch down too long and the screen restarts before you release it, you can just push RESET alone to cold start. Accidently hitting the FREEZER switch while you are running will lock your computer, but as long as it is not active when you RESET, it will not FORCE a cold start. You may get one anyway if your program is designed to produce one, so mount the switch in a protected spot. A little practice will get you a cold start every time. Installation requires some soldering and cutting, so don't try this if you haven't had experience. You will need a 74HC86 IC, a small push button switch and a 1/4 watt resistor between 1K and 30K. Take your 1200XL, 800XL or 130XE apart and locate the GTIA chip. (1200XL=U19:800XL=U17:130XE=U17) You need to isolate pin 11 of the GTIA from the rest of the circuit by cutting the wiring on the printed circuit board. The 130XE requires two cuts and an added wire since the pin is between two points that you would like to keep connected. The normal circuit is: pin 14 (RD5) of cartridge - pin 8 of MMU - resistor to ground - pin 11 of GTIA. In that example, you could cut the wiring to pin 11 and not remove any of the connection points from the circuit except pin 11. In the 130XE, pin 11 is between the cartridge and the MMU, so you have to restore the wiring from cart. to MMU after cutting out the pin.
1200XL: cut the trace on top of the board just to the left of Q4. MMU is U14.
800XL: cut the trace just below pin 11 on the GTIA chip (U17). MMU is U3
130XE: cut the trace on top of the board just below pin 11 of the GTIA chip (U17). Also cut the trace on the bottom of the board right next to pin 20 of U17. Add a wire from the pad near the last cut you made (near pin 20 of the GTIA chip) to pin 8 of the MMU chip (U3). This added wire restores the circuit between the cartridge and the MMU.
All machines: the added circuit is 1/4 of a 74HC86, which is an exclusive-or circuit. Wire pins 4,5,7,9,10,12, and 13 to ground. Connect pin 14 to +5v. Pin 1 goes to pin 8 of the MMU and pin 3 to pin 11 of GTIA. Connect a 1/4 watt resistor (1K-30K) from pin 2 to ground. Finally, mount the push button (normally open) switch on a clear area of your case and wire one side to +5v and the other to pin 2 of the '86. That is all that's needed. If you want to restore your machine to normal, solder a wire between pin 11 of GTIA and pin 8 of the MMU and remove the added IC and switch.
Well, it looks like it will take you longer to read this whole thing than it will take to build it. Just take your time and ASK FOR HELP if you aren't
sure!!
Yep!! May be a good idea to install MORRAM and FREEZER at the same time and on
the same board. (takes two ICs - maybe you should leave a little extra space
for.......?)
Bob Woolley

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