81 lines
3.3 KiB
Plaintext
81 lines
3.3 KiB
Plaintext
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.BP
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.SN 2
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.S1 MEMORY
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The EM machine has two distinct address spaces,
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one for instructions and one for data.
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The data space is divided up into 8-bit bytes.
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The smallest addressable unit is a byte.
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Bytes are numbered consecutively from 0 to some maximum.
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All sizes in EM are expressed in bytes.
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.P
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Some EM instructions can transfer objects containing several bytes
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to and/or from memory.
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The size of all objects larger than a word must be a multiple of
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the wordsize.
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The size of all objects smaller than a word must be a divisor
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of the wordsize.
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For example: if the wordsize is 2 bytes, objects of the sizes 1,
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2, 4, 6,... are allowed.
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The address of such an object is the lowest address of all bytes it contains.
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For objects smaller than the wordsize, the
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address must be a multiple of the object size.
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For all other objects the address must be a multiple of the
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wordsize.
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For example, if an instruction transfers a 4-byte object to memory at
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location \fIm\fP and the wordsize is 2,
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\fIm\fP must be a multiple of 2 and the bytes at
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locations \fIm\fP, \fIm\fP\|+\|1,\fIm\fP\|+\|2 and
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\fIm\fP\|+\|3 are overwritten.
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.P
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The size of almost all objects in EM
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is an integral number of words.
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Only two operations are allowed on
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objects whose size is a divisor of the wordsize:
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push it onto the stack and pop it from the stack.
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The addressing of these objects in memory is always indirect.
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If such a small object is pushed onto the stack
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it is assumed to be a small integer and stored
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in the least significant part of a word.
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The rest of the word is cleared to zero,
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although
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EM provides a way to sign-extend a small integer.
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Popping a small object from the stack removes a word
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from the stack, stores the least significant byte(s)
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of this word in memory and discards the rest of the word.
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.P
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The format of pointers into both address spaces is explicitly undefined.
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The size of a pointer, however, is fixed for a member of EM, so that
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the compiler writer knows how much storage to allocate for a pointer.
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.P
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A minor problem is raised by the undefined pointer format.
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Some languages, notably Pascal, require a special,
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otherwise illegal, pointer value to represent the nil pointer.
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The current Pascal-VU compiler uses the
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integer value 0 as nil pointer.
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This value is also used by many C programs as a normally impossible address.
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A better solution would be to have a special
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instruction loading an illegal pointer value,
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but it is hard to imagine an implementation
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for which the current solution is inadequate,
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especially because the first word in the EM data space
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is special and probably not the target of any pointer.
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.P
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The next two chapters describe the EM memory
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in more detail.
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One describes the instruction address space,
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the other the data address space.
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.P
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A design goal of EM has been to allow
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its implementation on a wide range of existing machines,
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as well as allowing a new one to be built in hardware.
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To this extent we have tried to minimize the demands
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of EM on the memory structure of the target machine.
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Therefore, apart from the logical partitioning,
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EM memory is divided into 'fragments'.
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A fragment consists of consecutive machine
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words and has a base address and a size.
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Pointer arithmetic is only defined within a fragment.
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The only exception to this rule is comparison with the null
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pointer.
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All fragments must be word aligned.
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