H838024 datasheet(108/697 Pages) RENESAS

108 / 697 page

Section 2 CPU

Rev. 7.00 Mar 10, 2005 page 66 of 652

REJ09B0042-0700

2.9

Application Notes

2.9.1

Notes on Data Access

1. Access to Empty Areas:

The address space of the H8/300L CPU includes empty areas in addition to the RAM,

registers, and ROM areas available to the user. If these empty areas are mistakenly accessed

by an application program, the following results will occur.

Data transfer from CPU to empty area:

The transferred data will be lost. This action may also cause the CPU to misoperate.

Data transfer from empty area to CPU:

Unpredictable data is transferred.

2. Access to Internal I/O Registers:

Internal data transfer to or from on-chip modules other than the ROM and RAM areas makes

use of an 8-bit data width. If word access is attempted to these areas, the following results will

occur.

Word access from CPU to I/O register area:

Upper byte: Will be written to I/O register.

Lower byte: Transferred data will be lost.

Word access from I/O register to CPU:

Upper byte: Will be written to upper part of CPU register.

Lower byte: Unpredictable data will be written to lower part of CPU register.

Byte size instructions should therefore be used when transferring data to or from I/O registers

other than the on-chip ROM and RAM areas. Figure 2.17 shows the data size and number of

states in which on-chip peripheral modules can be accessed.

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