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hd6120: Rewrite notes

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AJR 2021-06-12 10:55:45 -04:00
parent fca8f89d10
commit 541d6416f8
1 changed files with 28 additions and 14 deletions
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src/devices/cpu/pdp8/hd6120.cpp
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@ -86,20 +86,34 @@
Register”), PC and the two stack pointers, include several which are
only implicitly used in execution: a TEMP register that latches ALU
outputs, the instruction register IR, and the output latch register OL
that holds all addresses and data to be output on the DX bus. HD-6120
also internally maintains a group of 3-bit registers whose path
connects to TEMP, each of which contains the current memory extension
fields, their mirrors or various flags. (This emulation extends the
field registers to 4 bits to include the CTRLFF, PDF and PEX flags,
which are neither readable nor output directly at any time.)
Particular flag registers are enabled on the C0, C1 and EMA2 lines
during the final writes of ISZ, DCA and JMS. The most important of
these flag registers includes LINK, the interrupt enable flip-flop and
the GT flag hitherto provided only on arithmetic extensions of some
previous PDP-8 CPUs, though like MQ it conveys no specific purpose
here. Another flag register contains the inverse of the active-low
INTREQ input, the PWRON flag (set if STRTUP = VSS at RESET time,
causing entry into panel mode) and 0 in its LSB.
that holds all addresses and data to be output on the DX bus.
HD-6120 also maintains a group of 3-bit internal registers whose data
path connects to TEMP. These are used to hold the current memory
extension fields, their mirrors and various flags. (This emulation
extends the field registers to 4 bits to include the CTRLFF, PDF and
PEX flags, which are neither readable nor output directly at any
time.) These 3-bit registers may be enabled on the C0, C1 and EMA2
lines at particular times, and the GTF, GCF, PRS, RDF, RIF and RIB
internal IOTs read various combinations of them into AC. They include:
MSB LSB Output conditions
-----------------------------------------
IF0 IF1 IF2 IFETCH, direct operands (except if FZ)
IB0 IB1 IB2 None (until transferred to IF)
ISF0 ISF1 ISF2 None
DF0 DF1 DF2 Indirect operand addressing, IOTs, etc.
DSF0 DSF1 DSF2 None
LINK GT IEFF DCA AC writes
INTREQ* PWRON 0 ISZ result writes
BTSTRAP PNLTRP HLTFLG JMS PC writes
The GT flag, like MQ, is not used for any specific purpose on the
HD-6120, unlike the arithmetic extensions of previous PDP-8 CPUs which
originally implemented them. The INTREQ flag is 1 when the input pin
is sampled active low and 0 when it is inactive. The PWRON flag is set
if STRTUP is sampled as VSS at RESET time; it causes the CPU to trap
into panel mode before executing its first instruction.
Undefined Group 3 OPRs and internal IOTs have no effect on the
HD-6120 except that both interrupts and panel requests are blocked