update README file

README.md

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 # cocostress
 CoCo3 RAM Stress Tester, for TRS-80 Color Computer 3
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+This is a diagnostic program which detects the amount of RAM installed in a CoCo 3 (either 128kb or 512kb up to 8MB) and then writes and reads memory pages, checking for any errors.  All memory pages are tested in a continuous loop.  The CoCo must be reset or powered down to stop the stress test.
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+When an error is encountered, the error type (I1-I3 or D1-D3), the memory location of the error (page and offset), and the byte values (written vs. read) are logged and displayed in a list on the screen. The latest 8 errors are shown, with the newest at the top.
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+Each byte of each page (except those which get overwritten by the code page or graphics display) is tested twice: first immediately after writing the page, and then a second time after testing all other pages, just before writing new values to the page. If the first (immediate) verify fails, an I-type error is logged. If the second (delayed) verify fails, a D-type error is logged. When a byte verify fails, that same byte is read once or twice more to determine if the error was transient or persistent. The number of failed reads (1-3) is appended to the error type (I or D).
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+As the test runs, this program shows the 8kb memory page number currently being tested ($000-$3FF for 8MB), the total amount of memory detected in kilobytes, the total number of megabytes of RAM tested since the program started, and the total number of errors detected.
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+To start the test, LOADM the 'STRESS12' program.
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