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CHAPTER 3 CPU
3.2 Dedicated Register
The dedicated register in the CPU consists of a program counter (PC), two arithmetic
operation registers (A and T), three address pointers (IX, EP, and SP), and program
status (PS) register. The size of each register is 16 bits.
■ Dedicated Register Configuration
The dedicated register in the CPU consists of seven 16-bit registers. Some registers allow only the lower 8
bits to be used.
Figure 3.2-1 shows the configuration of the dedicated register.
Figure 3.2-1 Configuration of Dedicated Register
■
Functions of the Dedicated Register
● Program counter (PC)
The size of the program counter is 16 bits. It indicates the memory address at which the CPU is currently
handling an instruction. The program counter is updated with an instruction executed, interrupt, or reset.
The initial value specified after the reset operation is the mode data read address (FFFD
H
).
● Accumulator (A)
The accumulator is a 16-bit arithmetic operation register. It handles arithmetic operations or data transfer
using data on memory or data in another register such as temporary accumulator (T). The accumulator
allows data in it to be used as a word (16 bits) or bytes (8 bits). When arithmetic operations or data transfer
is handled in the unit of a byte, only the lower 8 bits (AL) of the accumulator are used; the upper 8 bits
(AH) remain unchanged. The initial value specified after the reset operation is undefined.
16 bits
:
A:
T:
IX :
EP :
SP :
RP CCR :
PS
PC
Initial value
FFFD
H
Undefined
Undefined
Undefined
Undefined
Undefined
Program counter
Indicates the current instruction stored position.
Accumulator
Temporary register that handles arithmetic operations and
data transfer.
Temporary accumulator
Handles arithmetic operations together with the accumulator.
Index register
Indicates index address.
Extra-pointer
Indicates memory address.
Stack pointer
Indicates the current position in the stack.
Program status register
Stores the register bank pointer and condition code.
Flag I = 0
IL1 and IL0 = 11
The other bits are undefined.