244 lines
8.6 KiB
Plaintext
244 lines
8.6 KiB
Plaintext
.SN 5
|
|
.BP
|
|
.S1 "MAPPING OF EM DATA MEMORY ONTO TARGET MACHINE MEMORY"
|
|
The EM architecture is designed to be implemented
|
|
on many existing and future machines.
|
|
EM memory is highly fragmented to make
|
|
adaptation to various memory architectures possible.
|
|
Format and encoding of pointers is explicitly undefined.
|
|
.P
|
|
This chapter gives solutions to some of the
|
|
anticipated problems.
|
|
First, we describe a possible memory layout for machines
|
|
with 64K bytes of address space.
|
|
Here we use a member of the EM family with 2-byte word and pointer
|
|
size.
|
|
The most straightforward layout is shown in figure 2.
|
|
.Dr 40
|
|
65534 \-> |-------------------------------|
|
|
|///////////////////////////////|
|
|
|//// unimplemented memory /////|
|
|
|///////////////////////////////|
|
|
ML \-> |-------------------------------|
|
|
| |
|
|
| | <\- LB
|
|
| stack and local area |
|
|
| |
|
|
|-------------------------------| <\- SP
|
|
|///////////////////////////////|
|
|
|//////// inaccessible /////////|
|
|
|///////////////////////////////|
|
|
|-------------------------------| <\- HP
|
|
| |
|
|
| heap area |
|
|
| |
|
|
| |
|
|
HB \-> |-------------------------------|
|
|
| |
|
|
| global data area |
|
|
| |
|
|
EB \-> |-------------------------------|
|
|
| |
|
|
| program text | <\- PC
|
|
| |
|
|
| ( and tables ) |
|
|
| |
|
|
| |
|
|
PB \-> |-------------------------------|
|
|
|///////////////////////////////|
|
|
|////////// undefined //////////|
|
|
|///////////////////////////////|
|
|
0 \-> |-------------------------------|
|
|
.Df
|
|
Figure 2. Memory layout showing typical register
|
|
positions during execution of an EM program.
|
|
.De
|
|
.N 1
|
|
The base registers for the various memory pieces can be stored
|
|
in target machine registers or memory.
|
|
.IS
|
|
.N 1
|
|
.TS
|
|
tab(;);
|
|
l 1 l l l.
|
|
PB;:;program base;points to the base of the instruction address space.
|
|
EB;:;external base;points to the base of the data address space.
|
|
HB;:;heap base;points to the base of the heap area.
|
|
ML;:;memory limit;marks the high end of the addressable data space.
|
|
.TE 1
|
|
.IE
|
|
The stack grows from high
|
|
EM addresses to low EM addresses, and the heap the
|
|
other way.
|
|
The memory between SP and HP is not accessible,
|
|
but may be allocated later to the stack or the heap if needed.
|
|
The local data area is allocated starting at the high end of
|
|
memory.
|
|
.P
|
|
Because EM address 0 is not mapped onto target
|
|
address 0, a problem arises when pointers are used.
|
|
If a program pushed a constant, say 6, onto the stack,
|
|
and then tried to indirect through it,
|
|
the wrong word would be fetched,
|
|
because EM address 6 is mapped onto target address EB+6
|
|
and not target address 6 itself.
|
|
This particular problem is solved by explicitly declaring
|
|
the format of a pointer to be undefined,
|
|
so that using a constant as a pointer is completely illegal.
|
|
However, the general problem of mapping pointers still exists.
|
|
.P
|
|
There are two possible solutions.
|
|
In the first solution, EM pointers are represented
|
|
in the target machine as true EM addresses,
|
|
for example, a pointer to EM address 6 really is
|
|
stored as a 6 in the target machine.
|
|
This solution implies that every time a pointer is fetched
|
|
EB must be added before referencing
|
|
the target machine's memory.
|
|
If the target machine has powerful indexing
|
|
facilities, EB can be kept in a target machine register,
|
|
and the relocation can indeed be done on
|
|
every reference to the data address space
|
|
at a modest cost in speed.
|
|
.P
|
|
The other solution consists of having EM pointers
|
|
refer to the true target machine address.
|
|
Thus the instruction LAE 6 (Load Address of External 6)
|
|
would push the value of EB+6 onto the stack.
|
|
When this approach is chosen, back ends must know
|
|
how to offset from EB, to translate all
|
|
instructions that manipulate EM addresses.
|
|
However, the problem is not completely solved,
|
|
because a front end may have to initialize a pointer
|
|
in CON or ROM data to point to a global address.
|
|
This pointer must also be relocated by the back end or the interpreter.
|
|
.P
|
|
Although the EM stack grows from high to low EM addresses,
|
|
some machines have hardware PUSH and POP
|
|
instructions that require the stack to grow upwards.
|
|
If reasons of efficiency urge you to use these
|
|
instructions, then EM
|
|
can be implemented with the memory layout
|
|
upside down, as shown in figure 3.
|
|
This is possible because the pointer format is explicitly undefined.
|
|
The first element of a word array will have a
|
|
lower physical address than the second element.
|
|
.Dr 18
|
|
| | | |
|
|
| EB=60 | | ^ |
|
|
| | | | |
|
|
|-----------------| |-----------------|
|
|
105 | 45 | 44 | 104 214 | 41 | 40 | 215
|
|
|-----------------| |-----------------|
|
|
103 | 43 | 42 | 102 212 | 43 | 42 | 213
|
|
|-----------------| |-----------------|
|
|
101 | 41 | 40 | 100 210 | 45 | 44 | 211
|
|
|-----------------| |-----------------|
|
|
| | | | |
|
|
| v | | EB=255 |
|
|
| | | |
|
|
|
|
Type A Type B
|
|
.Df
|
|
Figure 3. Two possible memory implementations.
|
|
Numbers within the boxes are EM addresses.
|
|
The other numbers are physical addresses.
|
|
.De
|
|
.A 1 0
|
|
So, we have two different EM memory implementations:
|
|
.IS
|
|
.PS - 4
|
|
.PT A~\-
|
|
stack downwards
|
|
.PT B~\-
|
|
stack upwards
|
|
.PE
|
|
.IE
|
|
.P
|
|
For each of these two possibilities we give the translation of
|
|
the EM instructions to push the third byte of a global data
|
|
block starting at EM address 40 onto the stack and to load the
|
|
word at address 40.
|
|
All translations assume a word and pointer size of two bytes.
|
|
The target machine used is a PDP-11 augmented with push and pop instructions.
|
|
Registers 'r0' and 'r1' are used and suffer from sign extension for byte
|
|
transfers.
|
|
Push $40 means push the constant 40, not word 40.
|
|
.P
|
|
The translation of the EM instructions depends on the pointer representation
|
|
used.
|
|
For each of the two solutions explained above the translation is given.
|
|
.P
|
|
First, the translation for the two implementations using EM addresses as
|
|
pointer representation:
|
|
.DS
|
|
.TS
|
|
tab(:), center;
|
|
l s l s l s
|
|
_ s _ s _ s
|
|
l 2 l 6 l 2 l 6 l 2 l.
|
|
EM:type A:type B
|
|
|
|
|
|
LAE:40:push:$40:push:$40
|
|
|
|
ADP:3:pop:r0:pop:r0
|
|
::add:$3,r0:add:$3,r0
|
|
::push:r0:push:r0
|
|
|
|
LOI:1:pop:r0:pop:r0
|
|
::\-::neg:r0
|
|
::clr:r1:clr:r1
|
|
::bisb:eb(r0),r1:bisb:eb(r0),r1
|
|
::push:r1:push:r1
|
|
|
|
LOE:40:push:eb+40:push:eb-41
|
|
.TE
|
|
.DE
|
|
.P
|
|
The translation for the two implementations, if the target machine address is
|
|
used as pointer representation, is:
|
|
.N 1
|
|
.DS
|
|
.TS
|
|
tab(:), center;
|
|
l s l s l s
|
|
_ s _ s _ s
|
|
l 2 l 6 l 2 l 6 l 2 l.
|
|
EM:type A:type B
|
|
|
|
|
|
LAE:40:push:$eb+40:push:$eb-40
|
|
|
|
ADP:3:pop:r0:pop:r0
|
|
::add:$3,r0:sub:$3,r0
|
|
::push:r0:push:r0
|
|
|
|
LOI:1:pop:r0:pop:r0
|
|
::clr:r1:clr:r1
|
|
::bisb:(r0),r1:bisb:(r0),r1
|
|
::push:r1:push:r1
|
|
|
|
LOE:40:push:eb+40:push:eb-41
|
|
.TE
|
|
.DE
|
|
.P
|
|
The translation presented above is not intended to be optimal.
|
|
Most machines can handle these simple cases in one or two instructions.
|
|
It demonstrates, however, the flexibility of the EM design.
|
|
.P
|
|
There are several possibilities to implement EM on machines with
|
|
address spaces larger than 64k bytes.
|
|
For EM with two byte pointers one could allocate instruction and
|
|
data space each in a separate 64k piece of memory.
|
|
EM pointers still have to fit in two bytes,
|
|
but the base registers PB and EB may be loaded in hardware registers
|
|
wider than 16 bits, if available.
|
|
EM implementations can also make efficient use of a machine
|
|
with separate instruction and data space.
|
|
.P
|
|
EM with 32 bit pointers allows one to make use of machines
|
|
with large address spaces.
|
|
In a virtual, segmented memory system one could use a separate
|
|
segment for each fragment.
|