Hardware Reference
In-Depth Information
Now that the background is set, we begin by exploring how instruction set architectures can
be classified.
A.2 Classifying Instruction Set Architectures
The type of internal storage in a processor is the most basic differentiation, so in this section
we will focus on the alternatives for this portion of the architecture. The major choices are a
stack, an accumulator, or a set of registers. Operands may be named explicitly or implicitly:
The operands in a stack architecture are implicitly on the top of the stack, and in an accumu-
lator architecture one operand is implicitly the accumulator. The general-purpose register archi-
tectures have only explicit operands—either registers or memory locations. Figure A.1 shows
a block diagram of such architectures, and Figure A.2 shows how the code sequence C = A +
B would typically appear in these three classes of instruction sets. The explicit operands may
be accessed directly from memory or may need to be first loaded into temporary storage, de-
pending on the class of architecture and choice of specific instruction.
 
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