-*- Mode:TEXT; Package:USER -*-


		   Argument Descriptor Modularizations

			       A Proposal 
				  - - -

			      [Proposal #9]

			    Keith M. Corbett

			      Oct. 12, 1988 

				  - - -

A. INTRODUCTION

Argument descriptors have been implemented on the Lambda in a
processor-dependent fashion, and they are used quite widely throughout
the system.  The following is a proposal for modularizing the
implementation, thereby facilitating the Falcon port.

I suspect that Lambda argument descriptors are just the tip of this
particular iceberg.  There are many related parts of the system that use
system constants and microcode routines, and they may be handled in a
similar fashion.

B. BACKGROUND/CURRENT IMPLEMENTATION

An argument descriptor is a fixnum which is packed with information
describing how a function takes arguments.  The most common use is to
get the minimum and maximum number of arguments accepted by a function.
(Note that the current implementation allows at most 63 function
arguments.)

ARGS-INFO and %ARGS-INFO are both used in system code both return the
numeric argument descriptor for a given function. The LISP version
accepts function specs and invokes the microcode "%" version once it has
processed its argument.  

The following table of byte specs and bit field values is taken from the
file SYS:COLD;QCOM.LISP, which defines these constants for the processor
(they are loaded during the cold system build and are used by microcode
routines).  Constants with two percent signs prefixed (%%) are byte
specifiers for use with, e.g., LDB or DBP (and thus would have to change
for the Falcon).  Constants with one percent sign prefixed (%) are bit
field values for use with, e.g., LOGAND.

CONSTANT			VALUE (OCTAL)	COMMENTS
---------------------------------------------------------------------------
%ARG-DESC-QUOTED-REST		10000000	;has quoted rest argument
%%ARG-DESC-QUOTED-REST		(BYTE 1. 21.)

%ARG-DESC-EVALED-REST		40000000	;has evaluated rest argument
%%ARG-DESC-QUOTED-REST		(BYTE 1. 20.)

%ARG-DESC-ANY-REST		(BYTE 2. 20.)	;has either kind of rest argument  

%ARG-DESC-FEF-QUOTE-HAIR	20000000	;compiled fcn with hairy quoting
%%ARG-DESC-FEF-QUOTE-HAIR	(BYTE 1. 19.)	; must check ADL for full info

%ARG-DESC-INTERPRETED		10000000	;interpreted function, and/or
%%ARG-DESC-INTERPRETED		(BYTE 1. 18.)	; no information available
						; (VALUE = 1000077) ***

%ARG-DESC-FEF-BIND-HAIR		40000000	;compiled function with hairy
%ARG-DESC-FEF-BIND-HAIR		(BYTE 1. 17.)	; binding, linear enter must check
						; ADL

%ARG-DESC-MIN-ARGS		(BYTE 6. 6.)	;minimum number of required args
%ARG-DESC-MAX-ARGS		(BYTE 6. 0.)	; maximum number of required +
						; optional (all positional, not
						; rest) args

*** NOTE: Interpreted functions do not necessarily have this value.

C. SCOPE/WHERE ARGUMENT DESCRIPTORS ARE USED

The modules that utilize argument descriptors include:

  o Interpreter
  o Compiler
  o DESCRIBE (describing functions)
  o Macro-expansion (AUTOMATIC-DISPLACE)
  o Debugger / error handler (displaying/modifying frames)
  o Resources (getting arg-lists of parametizer functions)
  o Function typing (FUNCTIONP)

For my own convenience, I assembled a list of functions and files calling
%ARGS-INFO or ARGS-INFO, and collected these definitions (DEFUNs and fragments
thereof) into one file.  I verified this information by doing a Tags Search on
SYSTEM for "%ARG-DESC".

D. PORTABILITY RECOMMENDATIONS

1.) Fix QCOM to compute these values via calls to BYTE, if possible.  This
requires that a d ecision be made how QCOM type definitions (and cold loading
generally) will be handled for the Falcon system.

2.) Define WITH-ARGS-INFO (macro) and functions for getting the args-desc elements
with a single call to ARGS-INFO.

This facility is designed to minimize calls to AREGS-INFO, a goal of the existing
implemntation.

WITH-ARGS-INFO (MACRO): (FUNC VARLIST &BODY BODY)