Page 127 - A Practical Guide from Design Planning to Manufacturing

100 Chapter Four

another is not the operations they allow, but the way in which instruc-
tions specify the inputs and outputs of their instructions. Input and
output operands are implicit or explicit. An implicit destination means
that a particular type of operation will always write its result to the same
place. Implicit operands are usually the top of the stack or a special accu-
mulator register. An explicit destination includes the intended desti-
nation as part of the instruction. Explicit operands are general-purpose
registers or memory locations. Based on the type of destination operand
supported, architectures can be classified into four basic types: stack,
accumulator, register, or memory. Table 4-4 shows how these differ-
ent architectures would implement the adding of two values stored in
memory and writing the result back to memory.
Instead of registers, the architecture can define a “stack” of stored
values. The stack is a first-in last-out queue where values are added to
the top of the stack with a push instruction and removed from the top with
a pop instruction. The concept of a stack is useful when passing many
pieces of data from one part of a program to another. Instead of having
to specify multiple different registers holding all the values, the data is
all passed on the stack. The calling subroutine pushes as many values as
needed onto the stack, and the procedure being called pops, the appro-
priate number of times to retrieve all the data. Although it would be pos-
sible to create an architecture with only load and store instructions or with
only push and pop instructions, most architectures allow for both.
A stack architecture uses the stack as an implicit source and desti-
nation. First the values A and B, which are stored in memory, are pushed
on the stack. Then the Add instruction removes the top two values on
the stack, adds them together, and pushes the result back on the stack.
The pop instruction then places this value into memory. The stack archi-
tecture Add instruction does not need to specify any operands at all
since all sources come from the stack and all results go to the stack. The
Java Virtual Machine (JVM) is a stack architecture.
An accumulator architecture uses a special register as an implicit
destination operand. In this example, it starts by loading value A into
the accumulator. Then the Add instruction reads value B from memory
and adds it to the accumulator, storing the result back in the accumu-
lator. A store instruction then writes the result out to memory.

TABLE 4-4 Operands Types
Stack Accumulator Register Memory
Push A Ld A Ld R1, A Add C, B, A
Push B Add B Ld R2, B
Add St C Add R3, R2, R1
Pop C St C, R3