3-8 Chapter 3 CPU
MB89190/190A series
3.
2 Dedicated Registers
The CPU dedicated registers consist of the program counter (PC), two operation
registers (A, T), three address pointers (IX, EP, SP), and the program status (PS).
Each register is 16 bits long.
T Configuration of dedicated registers
The CPU dedicated registers consist of seven 16-bit registers. Some of these registers can use only the
lower 8 bits.
Figure 3.2 shows the dedicated register configuration.
Fig. 3.2 Configuration of Dedicated Register
T Functions of dedicated registers
Program counter (PC)
The program counter is a 16-bit counter indicating the memory address of the instruction that the CPU
is executing. The program counter contents are updated by instruction execution, interrupts, resets,
etc. The initial value at reset is the mode data read address (FFFDH).
Accumulator (A)
The accumulator is a 16-bit operation register performing various operations and transfers with data in
memory and data in other registers such as the temporary register (T). Data in the accumulator can
be processed as word length (16 bits) or byte length (8 bits). When byte-length operations or transfer
is performed, only the lower-order 8 bits (AL) of the accumulator are used and the higher-order 8 bits
(AH) remain unchanged. The initial value is undefined after reset.
Temporary accumulator (T)
The temporary accumulator is a 16-bit operation assistance register, and performs various operations
together with data in the accumulator (A). When the accumulator (A) performs operations using word-
length data (16 bits), data in the temporary accumulator is processed as word length.
When the
accumulator (A) performs operation using byte-length data (8 bits), data in the temporary accumulator
Initial value
16 bits
FFFD
H
PC
:
Program counter
Indicates current instruction location
Undefined
A
:
Accumulator
Temporary register used for operation and transfer
Undefined
T
:
Temporary accumulator
Performs operation together with data in accumulator
Undefined
IX
:
Index register
Indicates index addresses
Undefined
EP
:
Extra pointer
Indicates memory addresses
Undefined
SP
:
Stack pointer
Indicates current stack location
RP
CCR
:
Program status
I-flag = 0,
IL1,0 = 11
Other bits are
undefined.
PS
Register saving register bank pointers and condition code