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Alto Hardware Manual
Section 9: Nuts and Bolts for the Microcoder
62
9.0 NUTS AND BOLTS FOR THE MICROCODER
9.1 Standllrd Microcode Conventions
The microassembler which assembles microcode for the Alto is called Mu. By convention, microcode
source files have the extension .MU, and binary files have the extension .MB.
Standard Alto I ROM
microcode versions will be called AltoCodex.MU; those for Alto II will be called AltoIICodex.Mu. A
microcode source file can be divided into three largely separable pieces: the language definitions, which
tell Mu what names will be used for what octal values of what microcode fields; the constant definitions,
which declare all constants that may later be referenced, and which cause the constant memory to be laid
out;
and the register declarations, microinstruction" label declarations, and microinstructions.
In order for microprograms written to execute in the RAM to be compatible with those in the ROM, at a
minimum the constants assumed by the RAM microcode must be a subset of those declared by the ROM
microcode, and the subset must reside in the same addresses. As a practical matter, one should preface
one's
RAM
microcode by the same constant definitions which were used in the assembly of one's ROM
microcode.
In order to facilitate and encourage this compatibility, the file AltoConstsx.MU will be
maintained (the x corresponding to the latest AltoCodex) containing definitions and constants for both
Alto I and Alto II.
These can be logically incorporated into other microcode assemblies via the
"include" feature of Mu (# AltoConstsx.Mu;).
If one or more microcode tasks pass control back and forth between ROM and RAM, it becomes necessary
to associate addresses with microinstruction labels. It is possible to do this completely generally, based
on the microcode version number. A more limited solution is simply to fix the addresses of certain
useful labels. The following addresses are guaranteed in all standard Alto I microcode versions after 20,
and all standard Alto II microcode versions (and are included ill· AltoConstsx.Mu): .
ADDRESS
20B
37B
22B
l05B
l06B
120B
121B
124B
160B
777B
LABEL
START
TRAP!
RAMCYCX
BLT
BLKS
MUL
DIY
BITBLT
LO
SWRET
SEMANTICS
Beginning of emulator's main loop; starts a new emulated
instruction.
RAM
location to which unfamiliar traps are sent; ROM location
which implements trap sequence.
Fast cyclic shift subroutine.
Block transfer subroutine.
Block store subroutine.
Multiply subroutine.
Divide subroutine.
BITBLT subroutine.
Cyclic shift dispatch table.
In ROM! only -- see below
A standard convention requires that location SWRET in ROM! have the following microcode:
SWRET:
SWMODE;
: START;
This sequence enables a program to discover whether ROM! exists, i.e., whether the Alto has the 2K
PROM option (see section 9.2.4).
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