Memories Are Made of This
Robert D. Randall 841 So. 50th St. Lincoln, NE 68510
This article should answer some of the questions you have about computer memory and how it works in the TRS-80. I will also take a look at expanding memory in the keyboard and the expansion interface, as well as what to do if your computer memory ever goes on the blink.
To begin, let's look at the computer memory of a few decades ago. Magnetic core memory was probably the most popular for small computers and also used as secondary or external memory for larger systems in the 1950's and 60's. Magnetic core memory was developed at the Massachusetts Institute of Technology for the Whirlwind I computer. It had an accesstime of a few microseconds, (millionths of a second), was reliable, and was somewhat less expensive when compared to other types of memory in use at that time. It was not uncommon to have memories in the range of a few thousand to 32,000 words of storage. The drawbacks to these systems were their large physical size, their high power consumption, and their high price tags.
The magnetic core was made up of ferrite material, usually in the shape of little doughnuts. These doughnut shaped core elements were a millimeter or so in diameter and were strung by the thousands on a grid of wires. To store data the ferrite core was magnetized to read data, the magnetized ferrite core was used to induce an electrical current.
Random Acess Memory
Large scale integration has revolutionized the design of computers and other electronic devices. This is the process whereby tens of thousands of tiny circuits are fabricated upon a tiny chip of silicon, at times no more than a quarter of an inch on a side. What this has meant to computers should beapparenteverytimeyou powerup your TRS-80.
If you own a TRS-80 with 16K memory, you will have eight 4116 16K RAM chips in your keyboard unit. 4116 is the model number of the RAM chip; 16K means the chip contains over 16,000 bits of storage; and RAM is an acronym for Random Access Memory.
RAM is a memory device that will write into or read from its storage any data the CPU/microprocessor requires, at any address the CPU specifies. The time required to do this is known as the RAM chips' access time, which is usually expressed in nanoseconds (NS). When you see an advertisement for extra memory, you will usually see a number like 450 ns or 250 ns after the chip model, meaning that the chip has an access time of 450 nanoseconds or 250 nanoseconds, which is fast when you think that a nanosecond is one billionth of a second. A nanosecond is to one second what one second is to 31.7 years.
There are eight RAM chips in the keyboard of the TRS-80 because the microprocessor chip is an eight-bit processor. This means that data is read into and from the chip eight bits at a time. In computer jargon, four bits equal one nybble and eight bits equal one byte. Twelve to sixteen bits equal a word.
Eight RAMs are necessary because only one bit may be addressed at a time in the 4116 RAM. In order to access eight bits of storage, you would need to access the
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