Radio Shack Computer

TRS80 Model II
$3,500 with 1 Drive

I didn't actually buy an IMSAI new (my Brother did), as my first computer was a Radio Shack TRS80 Model II. (Larger than the classic 'trash 80' that made Tandy famous.) The Model II featured a single 8" floppy integrated in with the monitor and motherboard. It used a double density Shugart 8" drive that would reliably format to 596k per disk. Additional drives could be added in a separate bay, and I introduced a 2-drive system with a NEC Spinwriter into my office in early 1980. I selected this hardware because it was "CP/M compatible" which was the buzzword of the day, though Radio Shack never supported CP/M preferring to promote their own proprietary TRSDOS. I used the CP/M and the newly released and bestselling WordStar.

TRS80 Model II
Expansion Drive
Radio Shack was much better known for their Model 1, Model 3 and Model 4 systems but these were all another story. Though based on the same Z80 processor they were not generally CP/M compatible. The reason for that was this. CP/M had a number of hard coded addresses within the operating system. The very first page of memory (0000h to 100h) was reserved for operating system purposes. All transient programs (user programs) were to be assembled and hardcoded to begin execution at 100h. The CP/M operating kernal would load in the top of the memory area, but it would always be entered by calls to the base page. The most famous operating system enter was "JMP 0005" although you could also use "CALL 0005" also. The problem with these proprietary radio shacks systems was that they had a ROM which used the basepage address space and also the address space assumed for the commencement of execution of the CP/M programs.

The Radio Shack ROM came to be there because the very first shack computer had the operating system in a ROM and the programmer simply started at address one and used the available space. Since bother the proprietary Shack system and CP/M used hard coded addressing, ou couldn't have both.

By 1980 CP/M was a raging success and Radio shack was caught with a propriety system which was co-dominate with Apple, but neither were CP/M. VisiCalc came along which the 'killer app' for the Apple, but no kiler apps appeared for the Radio Shack, and the action with the Z80 processor was with CP/M. This explained the Model 2. Tandy couldn't bring themselves to be completely hardware compatible so they did not support the S-100 bus, but they did provide full software compatibility with CP/M--which mostly required that the Address space not have a bunch of ROMS taking up low addresses. S-100 compatibility was not a mandate in the market and many manufacturers (most of whom are not forgotten) produced 'single board' computers where were CP/M compatible. There wasn't much to upgrade anyway. The Model II had a card cage and was not technicaly a single board computer. The cage was smaller than the S-100 cage, and it did ultimately allow thee systems to be upgraded to support the Motorola 68000 which was widely expected to be the successor to the Intel family of chips.

The Model II, sported a Z80 running at 4 mghz which was vastly faster than the 2 Mhz 8080 found in the IMSAI, Although Tandy did include a wait state to slow the processor down enough that it would not overun the RAM. As I recall my first system used 4k DRAM chips so 64K was a lot of chips, however 16K chips quickly became the norm. I could be wrong about this, It may have had 16k DRAMS.

The other good thing about the 8 inch drives on the Model II is that there was an 'IBM standard' for the single density format for the 8 inch floppy. This was a 241K format on one side of a floppy but it was standard. Every one and their pup and extended this format for more capacity by increasing the density but none of these were standard, as Digital Research (the maker of CP.M) did not provide the Bios and everyone implemented their bios in a different way. On 8 inch drives, however, the single density format was always supported which made for an interchange capability that we sort of take for granted today. by contrast no standard ever existed for the format of a minifloppy (5.25 inch floppy) under CP/M and there were over 2 dozen formats used, (a different one for every manufacturer almost) and none was universal.

Though the original TRS80 Model II contained only a single full height floppy drive, It had provision for plugging in an expansion chassis which allowed support for up to 3 more floppy drives for a total of 4. This relatively large storage capacity, (596k per single sided drive) made this unit popular for accounting systems. This Model first became available in the spring of 1980 and began to be replaced by the Model 12 during the summer of 1983.

The Model 12 was different internally in that it was a single board computer with an optional card cage, but the real advantage of the the Model 12 is that it used the new 'half high' floppy drives allowing 2 double sided drives to stand beside the screen each with twice the capacity of the single sided ones in the Model II. At 1.2 Megs per floppy, this allowed a 2 drive single unit to provide the storage capacity which previously required a 4 drive Model II.

Radio Shack Links

The Following links may be of interest if you found this Radio Shack page interesting.

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