
ST72321xx-Auto Central processing unit (CPU)
Doc ID 13829 Rev 1 33/243
5 Central processing unit (CPU)
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 six CPU registers shown in Figure 7 are not present in the memory mapping and are
accessed by specific instructions.
Figure 7. CPU registers
Accumulator
X index register
Y index register
Stack pointer
Condition code register
Program counter
70
1C1I1HI0NZ
Reset value = reset vector @ FFFEh-FFFFh
70
70
70
0715 8PCH PCL
15 8 7 0
Reset value = stack higher address
Reset value = 1 X11X1XX
Reset value = XXh
Reset value = XXh
Reset value = XXh
X = undefined value
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