SERIES ARE cOPYRIGHT (c) 1989, 1990 wESLEY j. wIESE. aLL RIGHTS RESERVED. aNY AND ALL CONSEQUENCES RESULTING FROM THE USE OF THESE DOCUMENTS IS THE EXPRESSED RESPONSIBILITY OF THE USER.tHE AUTHOR WILL NOT BE HELD LIABLE AND DOES NOT IMPLY ANY WARRANTY OR GUARANTEE. tHE AUTHOR DOES CERTIFY THAT ALL INFORMATION PRESENTED IN THESE DOCUMENTS WAS CORRECT, TO HIS KNOWLEDGE, AT THE TIME OF DISTRIBUTION OF THESE DOCUMENTS.
aLL DOCUMENTS IN THIS
@ dENSITY lEVEL dISPLAY
SERIES ARE THE SOLE PROPERTY OF THE AUTHOR, WHO RETAINS ALL COPYRIGHTS AND PATENTS TO THE MATERIAL PRESENTED.
aLL DOCUMENTS IN THIS
@dENSITY lEVEL dISPLAY
SERIES WILL NOT BE DISTRIBUTED OR REPRODUCED OR STORED IN ANY RETRIEVAL SYSTEM WITHOUT THE PRIOR PERMISSION OF THE AUTHOR.
tHIS DOCUMENT APPEARED FIRST IN THE sOUTHERN mAINE cOMMODORE uSER'S gROUP (
@s.m.c.u.g
.) NEWSLETTER,
@c-link
, vOLUME 6, iSSUE 5, mAY 1989.
fIRST LET'S LOOK AT THE WAY cOMMODORE dos USES DENSITYS FOR gcr (gROUP cODED rECORDING). uSING A PROGRAMMABLE DIVIDE BY n COUNTER OFF OF A 16 mEGAHERTZ CLOCK, THE SPEED AT WHICH THE DATA IS WRITTEN TO (OR READ FROM) A DISK CAN BE ALTERED. tHIS IS NEEDED BECAUSE THE TRACKS GET PHYSICALLY SMALLER TOWARDS THE CENTER OF THE HUB SO THE FREQUENCY AT WHICH THE DATA IS BEING WRITTEN TO OR READ FROM THE DISK HAS TO BE DECREASED TO KEEP SOME NORMALCY TO THE FLUX DENSITY OF THE DISK. nORMALLY THERE ARE FOUR DENSITYS, ONE BEING THE MOST DENSE, FOUR THE LEAST. mANY COMMERCIAL PROGRAMS USE ONLY THE FIRST TWO DENSITYS TO CRAM MORE INFORMATION ON THE HIGHER TRACKS. uSING A SMALL INTERNALLY OR EXTERNALLY MOUNTED HARDWARE DEVICE WE CAN READ THE DENSITY, VIA 7-SEGMENT led DISPLAY, THAT IS BEING WRITTEN TO OR READ FROM THE DISK.
uSING THE 16 mEGAHERTZ CRYSTAL CLOCK MODULE AS A BASE REFERENCE, THE DIVIDE BY n COUNTER CAN BE ALTERED VIA SOFTWARE TO SPEED UP OR SLOW DOWN THE CLOCK FREQUENCY AT WHICH EACH BYTE OF DATA IS TRANSFORMED INTO 8 BIT SERIAL DATA THEN PASSED TO THE WRITE AMPLIFIERS. tHIS FREQUENCY VARIES FROM 1.2307 mEGAHERTZ ON THE OUTER TRACKS (DENSITY 1) TO 1 mEGAHERTZ ON THE INNER TRACKS (DENSITY 4). tHE SOFTWARE LOCATION FOR THIS CONTROL IS 1c00 hEX IN THE DRIVE, BITS 5 AND 6. tHIS IS pORT b OF ONE OF THE 6522'S via (
ERSATILE
NTERFACE
DAPTOR) CHIP. tHE dos (
PERATING
YSTEM) DEDICATES THIS CHIP TO TRANSFERRING DATA TO AND FROM THE THE DISK. iT ALSO HAS HARDWARE CONTROL OF THE DISK DRIVE MOTOR, DENSITY LEVELS, STEPPER MOTOR, SYNC BYTES, WRITE PROTECT SENSOR AND DRIVE ACTIVITY led. tHE
@dENSITY lEVEL dISPLAY
IS A PIECE OF HARDWARE THAT UTILIZES BITS 5 AND 6 OF PORT b OF THIS 6522 via. iT MONITORS THESE BITS INDEPENDENT OF THE SOFTWARE CONTROLLING THEM, WHETHER THAT BE cOMMODORE dos OR A COMMERCIALLY PROTECTED PROGRAM. tHE DEVICE CONSISTS OF A nOR GATE, AN eXCLUSIVE-OR GATE, A bcd DECODER/DRIVER, 330 @ RESISTORS AND A 7-SEGMENT led. bIT MANIPULATION IS NEEDED TO TRANSPOSE THE 2 BITS OF DATA INTO A EASILY READABLE FORM.
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