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; Part of Scheme 48 1.9. See file COPYING for notices and license.
; Authors: Richard Kelsey, Jonathan Rees, Marcus Crestani
; Essential constants for 32-bit platforms.
; Fundamental parameters
(define bytes-per-cell 8)
(define log-bytes-per-cell 3)
(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 62) ; in Scheme48
(define c-useful-bits-per-word 64) ; 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)
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