88 lines
2.5 KiB
Scheme
88 lines
2.5 KiB
Scheme
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; -*- Mode: Scheme; Syntax: Scheme; Package: Scheme; -*-
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; Copyright (c) 1993-1999 by Richard Kelsey and Jonathan Rees. See file COPYING.
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; This is file memory.scm.
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; Fundamental parameters
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(define bits-per-byte 8)
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(define bytes-per-cell 4)
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(define bits-per-cell (* bits-per-byte bytes-per-cell))
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(define (bytes->cells bytes)
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; using shift instead of quotient for speed
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; (quotient (+ bytes (- bytes-per-cell 1)) bytes-per-cell)
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(arithmetic-shift-right (+ bytes (- bytes-per-cell 1))
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2)) ; log(bytes-per-cell)
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(define (cells->bytes cells)
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(* cells bytes-per-cell))
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; Addresses
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;
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; An "addressing unit" is the smallest quantum of storage addressed by
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; an address on a particular machine. On a DEC-20, 3600, or other
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; word-addressed architecture there is one addressing unit per cell. On
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; the VAX or 68000, though, the addressing unit is the byte, of which there
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; are 4 to a cell.
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;
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; Note: by a "byte" is meant enough bits to store either a character or
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; a bytecode. That probably means either 7, 8, or 9 bits.
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;
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; If the addressing unit is smaller than a cell each address will have some
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; number of "unused bits" at its low end. On a byte-addressable machine with
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; 32 bit addresses, there are two.
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(define unused-field-width 2)
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(define addressing-units-per-cell 4)
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(define (cells->a-units cells)
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(adjoin-bits cells 0 unused-field-width))
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(define (a-units->cells cells)
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(high-bits cells unused-field-width))
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(define (bytes->a-units byte-count)
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(cells->a-units (bytes->cells byte-count)))
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(define (address1+ x)
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(address+ x addressing-units-per-cell))
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(define (address2+ x)
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(address1+ (address1+ x)))
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; Memory access
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(define *memory*)
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(define *memory-begin* 0)
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(define *memory-end* 0)
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(define (memory-begin)
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*memory-begin*)
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; Size of memory in cells.
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(define (memory-size)
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(a-units->cells (address-difference *memory-end* *memory-begin*)))
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(define (create-memory size initial-value) ;size in cells
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(let ((size (cells->a-units size)))
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(cond ((not (= size (address-difference *memory-end* *memory-begin*)))
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(if (not (= *memory-end* 0))
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(deallocate-memory *memory*))
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(set! *memory* (allocate-memory size))
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(if (= -1 *memory*)
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(error "out of memory, unable to continue"))
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(set! *memory-begin* *memory*)
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(set! *memory-end* (+ *memory* size))))))
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(define fetch word-ref)
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(define fetch-byte unsigned-byte-ref)
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(define store! word-set!)
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(define store-byte! unsigned-byte-set!)
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(define fetch-string char-pointer->string)
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(define fetch-nul-terminated-string char-pointer->nul-terminated-string)
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