Hardware Reference
In-Depth Information
illustrated in Fig. 4-10. In our implementation, it is convenient to
think of the operand stack as part of the local variable frame. In any
case, an implicit register contains the address of the top word of the
stack. Notice that, unlike
CPP
and
LV
, this pointer,
SP
, changes dur-
ing the execution of the method as operands are pushed onto the stack
or popped from it.
4.
The method area.
Finally, there is a region of memory containing the
program, referred to as the ''text'' area in a
UNIX
process. An
implicit register contains the address of the instruction to be fetched
next. This pointer is referred to as the Program Counter, or
PC
.
Unlike the other regions of memory, the method area is treated as a
byte array.
SP
Current
Operand
Stack 3
Current
Local
Variable
Frame 3
LV
Local
Variable
Frame 2
Constant
Pool
Local
Variable
Frame 1
Method
Area
PC
CPP
Figure 4-10.
The various parts of the IJVM memory.
One point needs to be made regarding the pointers. The
CPP
,
LV
, and
SP
regis-
ters are all pointers to
words
, not
bytes
, and are offset by the number of words. For
the integer subset we have chosen, all references to items in the constant pool, the
local variables frame, and the stack are words, and all offsets used to index into
these frames are word offsets. For example,
LV
,
LV
+1,and
LV
+ 2 refer to the first
three words of the local variables frame. In contrast,
LV
,
LV
+4,and
LV
+ 8 refer to
words at intervals of four words (16 bytes).
In contrast,
PC
contains a byte address, and an addition or subtraction to
PC
changes the address by a number of bytes, not a number of words. Addressing for
PC
is different from the others, and this fact is apparent in the special memory port
provided for
PC
on the Mic-1. Remember that it is only 1 byte wide. Increment-
ing
PC
by one and initiating a read results in a fetch of the next
byte
. Incrementing
SP
by one and initiating a read results in a fetch of the next
word
.
