INSIGHT: Atari

Bill Wilkinson

Many people have asked me to discuss the use of the DOS 2.5 RAM disk with Atari computers other than the 130XE. Most are interested either in one of the 800XL memory upgrade kits now available or in simply using the extra 16K memory of an XL as a very small RAM disk. Since I've seen the subject treated incorrectly in several user group newsletters, I decided that some mildly technical discussion here would not be amiss.

Many months ago, in one of my columns, I described the memory map of an Atari XL computer. This time, let's see how a 130XE is a fairly simple expansion of the XL models.

An Atari 130XE has 126K—not 128K—of Random Access Memory. (Keep in mind that one kilobyte equals 1,024 bytes.) The first 62K is used and accessed exactly the same way as the 62K in the 1200XL and 800XL (that 62K is not a typo, either—more on this later). Now, a 6502 microprocessor can address a total of only 64K of contiguous memory, because the address counter goes from 0 to 65535 (64 * 1024). In the hexadecimal (base 16) numbering system used by computers, those addresses are expressed as $0000 (0) to $FFFF (65535). When the address counter passes $FFFF, it rolls back to $0000 again. This is kind of like a car speedometer which only goes to 99,999.9 miles; another tenth of a mile and you have a new car again.

So, how does the 130XE access its extra 64K of memory? By a technique known as bank selection.

Cashing In At The Memory Bank

The extra 64K in the 130XE is divided into four separate 16K banks. The 6502 can access only one of these banks at a time. But wait, you say, if the main memory uses up the full addressable range of the 6502, where do these extra banks fit in?

The answer. 16K of the main memory (that is, of the regular 64K) is disabled. Effectively, then, a 130XE has five banks of RAM, each consisting of 16K, plus another 46K (not a typo) which is not bank selected.

Now comes the important part: Just where, within the 64K address space of the 6502, are these five banks addressed? As Appendix H of the 130XE owner's manual states, the selectable 16K bank falls between locations 16384 (hex $4000) and 32767 ($7FFF). This is the second quarter of the 6502's 64K memory space. Why was this 16K area chosen instead of some other area? Because the first quarter of memory includes zero page, and bank-selecting zero page is a tricky proposition in a computer which is handling interrupts. The other two quarters of memory share space with cartridges and/or the operating system ROM, which would make programming more complicated. Thus, the second quarter of memory wins by default.

Okay so far. Now let's consider where BASIC programs reside in memory. Generally they begin at a memory location called LOMEM, which can vary but is usually between $1C00 and $2400 (about 7000 and 9000) when DOS is booted. BASIC programs always end below screen memory, which in turn is below the BASIC ROM. In practice, this means that BASIC programs and their variables are limited to a length of about 31K.

Let's assume that LOMEM is at $2000 (8192). Let us also assume that we have loaded or typed in a BASIC program which is 12,000 bytes long. Where does that program end? Smack dab in the middle of the second quarter of memory, where the banks are selected.

You might think that this would cause a problem on a 130XE, since it has to switch that bank of memory on and off. But it's not a problem, because one of those five banks is assigned to be main memory—that is, the memory corresponding to the only memory at that address in a 1200XL or 800XL. The DOS 2.5 RAM disk never touches that bank; it limits itself to the other four banks.

Okay, enough background on the 130XE. Is there a way to use the extra 16K memory of the 800XL as a RAM disk? Yes, but it isn't easy. That extra memory is addressed from $C000 to $FFFF (but see below for an exception). Aside from the fact that DOS 2.5 wasn't designed to see a RAM disk in this address range, this range is shared with the operating system ROMs and the hardware input/output area. Shared? Yep, more bank selection. And this bank is even trickier to use.

To Be Continued

Just as things start to get interesting, I run out of room. There is much more to this topic. For example, we haven't even looked for the missing 2K of RAM in the XLs and XEs, have we? And wouldn't it be nice to consider the effects of some of the add-on memory kits for the XLs? Until next month, let me tantalize you with some tidbits.

The RAM disk which emulates drive 8 (D8:) is one of the nice features of DOS 2.5. One of the not-so-nice features is that the RAM disk is always D8:. Many, many programs which want two disk drives assume that the second drive is D2:. Wouldn't it be nice if we could change the RAM disk's drive number? Say no more. The BASIC program listings below accomplish this for you.

Program 1, "REPLACE.BAS," is for use with the RAMDISK.COM program supplied with DOS 2.5. After you boot the system with DOS 2.5 and RAMDISK.COM, this program simply changes all the magic memory locations in DOS so that the RAM disk is now addressed as D2:, (Or you can change lines 190 and 260 to make the RAM disk emulate any drive from D2: to D8:.) If you use Program 1, the DOS files DUP.SYS and MEM.SAV will be on D2:, but otherwise DOS 2.5 will be unchanged.

Program 2, "MAKERAM.BAS,” serves another purpose. As you've probably noticed, DUP.SYS and MEM.SAV take up a lot of room on the RAM disk. True, keeping them on the RAM disk does make DOS easier to use. However, if your program won't use DOS but could use more RAM disk space, why not leave them on D1:? That's exactly what MAKERAM.BAS does. It initializes and installs the RAM disk, but copies no files to it— all 499 RAM disk sectors are available for your use. Naturally, you may choose any drive number for the RAM disk (see lines 190 and 260 again). And, although we could change this program to allow it to work after RAMDISK.COM has booted, it is a waste of time since this program reinitializes the RAM disk, anyway. Therefore, you should erase or rename the RAMDISK.COM file when using MAKERAM.BAS (but don't erase your only copy of RAMDISK.COM).

Finally, Program 3, "MAKERAM.SUB," simply changes Program 2 into a subroutine which you can include in your own programs. Use it anytime you want your program to initialize a blank RAM disk.