Dedicated Registers - Fujitsu 8FX Hardware Manual

CHAPTER 2 CPU

2.1 Dedicated Registers

2.1 Dedicated Registers
The CPU has dedicated registers: a program counter (PC), two registers for
arithmetic operations (A and T), three address pointers (IX, EP, and SP), and the
program status (PS) register. Each of the registers is 16 bits long. The PS
register consists of the register bank pointer (RP), direct bank pointer (DP), and
condition code register (CCR).
■ Configuration of Dedicated Registers
The dedicated registers in the CPU consist of seven 16-bit registers. As for the accumulator (A)
and the temporary accumulator (T), using only the lower eight bits of the respective registers is
also supported.
Figure 2.1-1 shows the configuration of the dedicated registers.
Initial value
0xFFFD
0x0000 AH
0x0000 TH
0x0000
0x0000
0x0000
0x0030 RP
■ Functions of Dedicated Registers
●
Program counter (PC)
The program counter is a 16-bit counter which contains the memory address of the instruction
currently executed by the CPU. The program counter is updated whenever an instruction is
executed or an interrupt or a reset occurs. The initial value set immediately after a reset is the
mode data read address (0xFFFD).
●
Accumulator (A)
The accumulator is a 16-bit register for arithmetic operation. It is used for a variety of
arithmetic and transfer operations of data in memory or data in other registers such as the
temporary accumulator (T). The data in the accumulator can be handled either as word (16-bit)
data or byte (8-bit) data. For byte-length arithmetic and transfer operations, only the lower
eight bits (AL) of the accumulator are used with the upper eight bits (AH) left unchanged. The
initial value set immediately after a reset is "0x0000".
4
Figure 2.1-1 Configuration of Dedicated Registers
16 bits
PC : Program counter
Indicates the address of the current instruction.
AL : Accumulator (A)
Temporary storage register for arithmetic operation and transfer
TL : Temporary accumulator (T)
Performs arithmetic operations with the accumulator.
IX : Index register
Indicates an index address.
EP : Extra pointer
Indicates a memory address.
SP : Stack pointer
Indicates the current stack location.
DP CCR : Program status
Stores a register bank pointer,
PS
a direct bank pointer, and a condition code.
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