BRACKENRIDGE@USC-ISIB@sri-unix.UUCP (10/06/83)
From: Billy <BRACKENRIDGE@USC-ISIB> The Tall Tree board uses very little power. It is barely warm to the hand when powered up. The only place you are likely to get into power consumption problems is by adding a hard disk to a regular PC without an upgraded power supply. Dynamic RAM chips and associated circuitry just don't use much power; so this should be true for anybody's memory board. The problem you have posed is that you have eaten up all the address space in the IBM-PC. As we have built several hardware devices for the PC here at ISI, we have run into the problems of trying to live with the Tall Tree boards and their appetite for address space. The following is a table of memory usage in a system with a JRAM card. Pages are 64K segments, so there are sixteen pages or segments in the IBM-PC address space. 0-9 Official Memory A-B Reserved for display address space C Partially used for hard disk boot ROM D JRAM Swap area E JRAM Page map table F BIOS-BASIC ROM Official memory can be either fixed memory such as an AST or Microsoft card, or it can be a segment of JRAM memory permanently mapped into one of these pages. At cold boot time the IBM ROM diagnostics will search these ten pages of memory to run a memory check dependent on sense switch setting. This is why a large memory system takes so long to boot. When the JBOOT program runs it will permanently map in as many pages of JRAM as are necessary to give DOS 2.0 a specified amount of memory. If there is more non JRAM memory than was specified to JBOOT, JDRIVE will use this memory for an electronic disk. If the amount of non JRAM memory matches the switch settings, only JRAM memory will be used for electronic disk. The electronic disk contents will then be preserved over warm boots as the pages will not be accessable to the ROM memory diagnostics. The D page of memory is JDRIVE's swap area. JDRIVE will map the appropriate page into this address to accomplish the disk transfer simulation. A measure of memory protection is provided as pages of the electronic disk not mapped to the D page are inaccessable to any wandering program. Unfortunately the JRAM card uses the entire E page of address space for its page map registers. Tall Tree just didn't put the extra circuitry to completely decode the address lines, and thus they wasted this entire page of address space. Your configuration of 640K official memory plus (up to four) JRAM cards would completely fill up the PC address space. Currently our hardware boards are using space in the C segment avoiding the hard disk ROM. I know the Tecmar speech I/O board uses the entire E address page. This makes the Tecmar board incompatible with Tall Tree. Tecmar likes to advertise that their products have no address conflicts so long as you stay exclusively with Tecmar products, and as they make almost anything you might want in a PC this is one way to avoid the problem of address conflict.