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Alto Hardware Manual
Section 9: Nuts and Bolts for the Microcoder
63
9.2 Microcode Techniques Which Need Not Be Rediscovered
For the most part, since the Alto is such a simple machine, writing Alto microcode is a straightforward
exercise in rule-following. However, during the course of writing the few-odd thousand microinstructions
which have ever been written by anybody for the Alto, a few microcoding techniques have emerged as
particularly ingenious or useful or both.
They are recorded here for posterity.
The beginning microcoder is advised to acquire a copy of the standard microcode (AltoCodex.MU), and
to study it carefully in conjunction with this manual.
The knack comes easily.
9.2.1
Microcode Subroutines
You have probably already noticed that that the Alto hardware does not provide an easy way of doing
microcode-level subroutine calls and returns. Several subroutine-call techniques have evolved. Two of
these are used for RAM-tO-ROM subroutine calls, and these will be presented first.
PC CALL (used with BLT, BLKS, MUL, DIV, BITBLT)
This call takes advantage of the assumption that nobody in his right mind would want the
emulator to execute in the non-memory
1/0
area from 177000B to 177777B. Therefore when one
of these ROM subroutines terminates, the R-register PC is examined. If it is outside the range
177000B-177777B, then control is passed to the beginning of the emulator'S main loop in the
ROM.
Otherwise, control is passed to location PC AND 777B in RAM or ROMI.
The bank
dispatched to is determined by the SWMODE rules described in section 8.4.
Warning:
Some of these ROM subroutines modify PC during execution.
If BLT or BLKS or
BITBLT is terminated by an interrupt condition, PC is decremented by 1 so that the instruction
can be resumed later.
If a DIV is successful, PC is incremented by 1 to cause a skip.
REGISTER CALL (used with RAMCYCX)
This call uses an R-register, in this case CYRET (R-register 5), to dispatch into a table of successor
instructions. The cyclic shift subroutine, for example, is called from six places in the ROM. Each
of these places sets CYRET
to
the index of its successor instruction in the return dispatch table
[O-
S], and then dispatches into the cycle table beginning at
1.0.
The successor corresponding to
RAMCYCX dispatches into RAM or ROMlusing the low-order 10 bits of the PC register, according
to the SWMODE rules described in section 8.4.
.
IR
CALLS
These calls use the emulator'S IR register in various ways: some straightforward and some
devious.
The main advantages of IR calls are that
1) several levels of return can be encoded into a single number, because it is fairly easy
to dispatch on various parts of IR, and
2) unlike R-registers, IR can be loaded in one microinstruction.
The most straightforward use of IR is dispatching on its low-order 8 bits using the DISP bus
source. Since DISP is a bus source
)3,
a constant may be "and-ed" onto the bus with DISP,
allowing one to dispatch on sub-fields of DISP.
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