guile-prescheme/prescheme/platform.scm

;;; Ported from Scheme 48 1.9.  See file COPYING for notices and license.
;;;
;;; Port Author: Andrew Whatson
;;;
;;; Original Authors: Richard Kelsey, Jonathan Rees, Marcus Crestani, David Frese
;;;
;;;  scheme48-1.9.2/scheme/platform-interfaces.scm
;;;  scheme48-1.9.2/scheme/vm/data/platform-32.scm
;;;  scheme48-1.9.2/scheme/vm/data/platform-64.scm
;;;
;;; Essential constants for 32/64-bit platforms.
;;;

(define-module (prescheme platform)
  #:use-module (system base target)
  #:export (bytes-per-cell
            log-bytes-per-cell
            bits-per-byte
            bits-per-cell
            addressing-units-per-cell
            c-useful-bits-per-word
            s48-useful-bits-per-word
            unused-field-width
            tag-field-width
            data-field-width
            immediate-type-field-width
            pre-scheme-integer-size))

;; Fundamental parameters

(define 64bit? (>= (target-word-size) 8))

(define bytes-per-cell     (if 64bit? 8 4))
(define log-bytes-per-cell (if 64bit? 3 2))

(define bits-per-byte 8)
(define bits-per-cell (* bits-per-byte bytes-per-cell))
(define addressing-units-per-cell bytes-per-cell)

;; This is actually the mimimum for the different PreScheme implementations.
;; The Scheme version of PreScheme leaves 30 bits for PreScheme's fixnums.
;; There have to be enough bits to represent the largest fixnum in the system.
;; USEFUL-BITS-PER-WORD is not written in the image.
(define s48-useful-bits-per-word  (if 64bit? 62 30)) ; in Scheme48
(define c-useful-bits-per-word    (if 64bit? 64 32)) ; when compiled

;; Addresses
;;
;;  An "addressing unit" is the smallest quantum of storage addressed by
;;  an address on a particular machine.  On a DEC-20, 3600, or other
;;  word-addressed architecture there is one addressing unit per cell.  On
;;  the VAX or 68000, though, the addressing unit is the byte, of which there
;;  are 4 to a cell.
;;
;;  Note: by a "byte" is meant enough bits to store a bytecode.  That
;;  probably means either 7, 8, or 9 bits.
;;
;;  If the addressing unit is smaller than a cell each address will have some
;;  number of "unused bits" at its low end.  On a byte-addressable machine with
;;  32 bit addresses, there are two.

(define unused-field-width log-bytes-per-cell)

;; Descriptors
;;  A descriptor describes a Scheme object.
;;  A descriptor is represented as an integer whose low two bits are
;;  tag bits.  The high bits contain information whose format and
;;  meaning are dependent on the tag.
(define tag-field-width 2)
(define data-field-width (- bits-per-cell tag-field-width))

;; Immediates
;;  This streamlines 8-bit-byte-oriented implementations.
(define immediate-type-field-width
  (- bits-per-byte tag-field-width))

;; Pre-Scheme
(define pre-scheme-integer-size bits-per-cell)