RM0016 Central processing unit (CPU)
Doc ID 14587 Rev 8 23/449
1 Central processing unit (CPU)
1.1 Introduction
The CPU has an 8-bit architecture. Six internal registers allow efficient data manipulations.
The CPU is able to execute 80 basic instructions. It features 20 addressing modes and can
address six internal registers. For the complete description of the instruction set, refer to the
STM8 microcontroller family programming manual (PM0044).
1.2 CPU registers
The six CPU registers are shown in the programming model in Figure 1. Following an
interrupt, the registers are pushed onto the stack in the order shown in Figure 2. They are
popped from stack in the reverse order. The interrupt routine must therefore handle it, if
needed, through the POP and PUSH instructions.
1.2.1 Description of CPU registers
Accumulator (A)
The accumulator is an 8-bit general purpose register used to hold operands and the results
of the arithmetic and logic calculations as well as data manipulations.
Index registers (X and Y)
These are 16-bit registers used to create effective addresses. They may also be used as a
temporary storage area for data manipulations and have an inherent use for some
instructions (multiplication/division). In most cases, the cross assembler generates a
PRECODE instruction (PRE) to indicate that the following instruction refers to the Y register.
Program counter (PC)
The program counter is a 24-bit register used to store the address of the next instruction to
be executed by the CPU. It is automatically refreshed after each processed instruction. As a
result, the STM8 core can access up to 16 Mbytes of memory.
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