Mill Computing, Inc. Forums The Mill Architecture Instruction Encoding Reply To: Instruction Encoding

Ivan Godard
Post count: 627

From comp.arch, on how the Mill operation specification machinery works:
On 5/20/2016 8:04 PM, Ivan Godard wrote:

(in the “instruction decoding perfected” thread, to deprecate manual instruction bit-layout):

> In our spec machinery you supply traits for each encodable attribute.
> One of those traits tells the bit-allocator when the decoder will need
> the value. An attribute can be “pinned”, in which case it gets its own
> bitfield (which need not be contiguous) in every operation in the slot,
> needed or not; or “direct”, in which case it gets its own field, but
> that field is only in the ops that use that attribute and may be in
> different location in different ops; or “merged”, in which case it is
> cross-producted with other attribute to make a single larger value-set
> that is encoded as if it were an attribute itself; or “uncoded” in which
> case it doesn’t appear in the binary at all but is carried metainfo for
> the specification machinery.

People have asked for me to post Mill details occasionally. In case anyone is interested, as a p.s. on how specification driven layout works here’s a cut from the actual source code that declares the operation attributes for Mills. The enumerations for those attributes that are enums are declared elsewhere. An example is:
enum directionCode {
which attribute is used in shifts and a few other operations. As you see below, the value of this attribute is given (in conAsm) by the operation mnemonic rather than by an argument (“byMnemonic”); is encoded as part of a cross-product with other attributes (“merged”); and has the same meaning and potential valueset across all Mills (“universal”).

To add a new attribute the architect must declare the enum (if there is one), add a line for the attribute to the declares below, and add a traits specification for the possible values; here’s the traits for directionCode:

::insts[intTraits::count] =
(leftward, “l”,
“toward greater significance”),
(rightward, “r”,
“toward lesser significance”)

This gives the text that will be used in the mnemonic in conAsm to indicate the desired value (“l”/”r”), plus a short description that is used in online help and other documentation. For a simple attribute like this, the whole process takes around 15 minutes, including adding an operation that uses the attribute and rebuilding the world to generate the new binary encodings.

Here’s the specs for all instruction attributes used in the Mill as specified 2016/05/21.

declareEnumeratedAttrTraits(accessCode, byMnemonic, merged, universal);
declareEnumeratedAttrTraits(awarenessCode, byMnemonic, merged, universal);
declareNumericAttrTraits(base0Code, byParam, uncoded, byMember);
declareEnumeratedAttrTraits(basedCode, byMnemonic, merged, universal);
declareNumericAttrTraits(bit0Code, byParam, direct, byMember);
declareNumericAttrTraits(bit1Code, byParam, direct, byMember);
declareEnumeratedAttrTraits(blockCode, byMnemonic, uncoded, universal);
declareEnumeratedAttrTraits(ccGenCode, byMnemonic, uncoded, universal);
declareEnumeratedAttrTraits(conBytesCode, byDerivation, pinned, universal);
declareNumericAttrTraits(con0Code, byParam, uncoded, universal);
declareEnumeratedAttrTraits(condSenseCode, byMnemonic, merged, universal);
declareEnumeratedAttrTraits(conSet, byMnemonic, uncoded, universal);
declareNumericAttrTraits(count0Code, byParam, direct, byMember);
declareEnumeratedAttrTraits(directionCode, byMnemonic, merged, universal);
declareEnumeratedAttrTraits(domainCode, byMnemonic, merged, universal);
declareNumericAttrTraits(elem0Code, byParam, direct, byMember);
declareNumericAttrTraits(field0Code, byParam, direct, byMember);
declareEnumeratedAttrTraits(exactitudeCode, byMnemonic, merged, universal);
declareEnumeratedAttrTraits(exclusivityCode, byMnemonic, merged, universal);
declareNumericAttrTraits(extCount, byDerivation, pinned, universal);
declareNumericAttrTraits(fault0Code, byParam, merged, universal);
declareNumericAttrTraits(imm0Code, byParam, direct, bySlot);
declareNumericAttrTraits(lit0Code, byParam, pinned, byMember);
declareNumericAttrTraits(lit1Code, byParam, pinned, byMember);
declareNumericAttrTraits(lit2Code, byParam, pinned, byMember);
declareEnumeratedAttrTraits(memCode, byMnemonic, uncoded, universal);
declareNumericAttrTraits(memAttrCode, byParam, merged, universal);
declareEnumeratedAttrTraits(memorySubOpCode, byMnemonic, merged, universal);
declareNumericAttrTraits(NaRcode, byParam, direct, byMember);
declareEnumeratedAttrTraits(off0Code, byParam, direct, universal);
declareNumericAttrTraits(opand0Code, byParam, pinned, byMember);
declareNumericAttrTraits(opand1Code, byParam, pinned, byMember);
declareNumericAttrTraits(opand2Code, byParam, pinned, byMember);
declareNumericAttrTraits(opand3Code, byParam, pinned, byMember);
declareEnumeratedAttrTraits(opCode, byMnemonic, merged, universal);
declareEnumeratedAttrTraits(overflowCode, byMnemonic, merged, declareEnumeratedAttrTraits(scaleCode, byParam, direct, byMember);
declareNumericAttrTraits(scratchCode, byParam, direct, byMember);
declareNumericAttrTraits(specr0Code, byParam, direct, byMember);
declareNumericAttrTraits(specw0Code, byParam, direct, byMember);
declareNumericAttrTraits(streamr0Code, byParam, direct, byMember);
declareNumericAttrTraits(streamw0Code, byParam, direct, byMember);
declareNumericAttrTraits(tag0Code, byParam, direct, byMember);
declareNumericAttrTraits(trap0Code, byParam, merged, universal);
declareNumericAttrTraits(widthCode, byParam, direct, byMember);
declareNumericAttrTraits(pop0Code, byParam, direct, bySlot);
declareEnumeratedAttrTraits(resCount, byMnemonic, merged, universal);
declareEnumeratedAttrTraits(resultCode, byMnemonic, uncoded, universal);
declareEnumeratedAttrTraits(roundingCode, byMnemonic, merged, universal);
declareEnumeratedAttrTraits(shrinkCode, byMnemonic, merged, universal);
declareEnumeratedAttrTraits(vectorFill, byMnemonic, merged, universal);

Feel free to ask questions.

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