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