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2.5
Bank Addressing Types
In the bank method, the 16-Mbyte space is divided into 256 64-Kbyte banks. The
following five bank registers are used to specify the banks corresponding to each
space:
• Program counter bank register (PCB)
• Data bank register (DTB)
• User stack bank register (USB)
• System stack bank register (SSB)
• Additional data bank register (ADB)
■ Bank Addressing Types
●
Program counter bank register (PCB)
The 64-Kbyte bank specified by the PCB is called a program (PC) space. The PC space contains instruction
codes, vector tables, and immediate value data, for example.
●
Data bank register (DTB)
The 64-Kbyte bank specified by the DTB is called a data (DT) space. The DT space contains readable/
writable data, and control/data registers for internal and external resources.
●
User stack bank register (USB)/system stack bank register (SSB)
The 64-Kbyte bank specified by the USP or SSP is called a stack (SP) space. The SP space is accessed
when a stack access occurs during a push/pop instruction or interrupt register saving. The S flag in the
condition code register determines the stack space to be accessed.
●
Additional data bank register (ADB)
The 64-Kbyte bank specified by the ADB is called an additional (AD) space. The AD space, for example,
contains data that cannot fit into the DT space.
Table 2.5-1 lists the default spaces used in each addressing mode, which are pre-determined to improve
instruction coding efficiency. To use a non-default space for an addressing mode, specify a prefix code
corresponding to a bank before the instruction. This enables access to the bank space corresponding to the
specified prefix code.
After reset, the DTB, USB, SSB, and ADB are initialized to "00
specified by the reset vector. After reset, the DT, SP, and AD spaces are allocated in bank "00
to 00FFFF
), and the PC space is allocated in the bank specified by the reset vector.
H
CHAPTER 2 CPU
". The PCB is initialized to a value
H
" (000000
H
H
33
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