Specifications

ST72104Gx, ST72215Gx, ST72216Gx, ST72254Gx
14/141
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
■ 63 basic instructions
■ Fast 8-bit by 8-bit multiply
■ 17 main addressing modes
■ Two 8-bit index registers
■ 16-bit stack pointer
■ Low power modes
■ Maskable hardware interrupts
■ Non-maskable software interrupt
5.3 CPU REGISTERS
The six CPU registers shown in Figure 1 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)
In indexed addressing modes, these 8-bit registers
are used to create either effective addresses or
temporary storage areas for data manipulation.
(The Cross-Assembler generates a precede in
-
struction (PRE) to indicate that the following in-
struction refers to the Y register.)
The Y register is not affected by the interrupt auto-
matic procedures (not pushed to and popped from
the stack).
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 6. CPU Registers
ACCUMULATOR
X INDEX REGISTER
Y INDEX REGISTER
STACK POINTER
CONDITION CODE REGISTER
PROGRAM COUNTER
70
1C11HI NZ
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 =
1X11X1XX
RESET VALUE = XXh
RESET VALUE = XXh
RESET VALUE = XXh
X = Undefined Value