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ST72321 Datasheet(PDF) 20 Page - STMicroelectronics |
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ST72321 Datasheet(HTML) 20 Page - STMicroelectronics |
20 / 189 page ![]() ST72321 20/189 5 CENTRAL PROCESSING UNIT 5.1 INTRODUCTION This CPU has a full 8-bit architecture and contains six internal registers allowing efficient 8-bit data manipulation. 5.2 MAIN FEATURES ■ Enable executing 63 basic instructions ■ Fast 8-bit by 8-bit multiply ■ 17 main addressing modes (with indirect addressing mode) ■ Two 8-bit index registers ■ 16-bit stack pointer ■ Low power HALT and WAIT modes ■ Priority maskable hardware interrupts ■ Non-maskable software/hardware interrupts 5.3 CPU REGISTERS The 6 CPU registers shown in Figure 8 are not present in the memory mapping and are accessed by specific instructions. Accumulator (A) The Accumulator is an 8-bit general purpose reg- ister used to hold operands and the results of the arithmetic and logic calculations and to manipulate data. Index Registers (X and Y) These 8-bit registers are used to create effective addresses or as temporary storage areas for data manipulation. (The Cross-Assembler generates a precede instruction (PRE) to indicate that the fol- lowing instruction refers to the Y register.) The Y register is not affected by the interrupt auto- matic procedures. Program Counter (PC) The program counter is a 16-bit register containing the address of the next instruction to be executed by the CPU. It is made of two 8-bit registers PCL (Program Counter Low which is the LSB) and PCH (Program Counter High which is the MSB). Figure 8. CPU Registers ACCUMULATOR X INDEX REGISTER Y INDEX REGISTER STACK POINTER CONDITION CODE REGISTER PROGRAM COUNTER 70 1C 1I1 H I0 N Z RESET VALUE = RESET VECTOR @ FFFEh-FFFFh 70 70 70 0 7 15 8 PCH PCL 15 8 70 RESET VALUE = STACK HIGHER ADDRESS RESET VALUE = 1X 11X 1X X RESET VALUE = XXh RESET VALUE = XXh RESET VALUE = XXh X = Undefined Value |
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