HP97 The journey begins
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03-04-2021, 11:21 AM
(This post was last modified: 03-04-2021 11:41 AM by teenix.)
Post: #321
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RE: HP97 The journey begins
Hi all,
I thought this was an interesting snap shot. Traces are... 1) SYNC pulses. (About 310uS apart) which is the HP-97 instruction (56 bit) word time. 2) IaIs bus - which has the ROM instruction, ROM address, display data etc. 3) Data bus - X register in the stack = 0 4) Flag 2 - connects to PIK, CRC and ARC. It shows when the HP-97 code asks the PIK chip if a keypress is available. (pik1320 - First Sync pulse) You can see the pulse on the Flag 2 trace that came from the PIK which is telling the ARC that a key is available from the PIK's 7 key buffer. The 3rd instruction is a GOTO $079. This is actually stored in ROM as $079 which is the address where the HP-97 main code loop starts and normally jumps back there when a key is not pressed, among other things. The Sync line stayed LOW for this instruction because the previous instruction was an "IF" type (if s3 = 0). This tells the ARC that $079 is a ROM address to GOTO, not a code word to execute. However, because the Flag 2 pulsed HIGH, (which sets ARC Status Flag s3 = 1) this GOTO instruction is ignored. The instruction at the last sync pulse is the instruction that turns the display off while the key press is serviced. The S3 flag stays as is in the ARC until it is reset by a 0 -> s3 instruction which in this case follows the display off. Code:
cheers Tony |
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