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User’s Manual U12697EJ3V0UM
CHAPTER 26 ROM CORRECTION
26.1 ROM Correction Functions
µ
PD784224, 784225, 784224Y and 784225Y convert part of the program within the mask ROM into the program
within the internal expansion ROM.
The use of ROM correction enables command bugs discovered in the mask ROM to be repaired, and change the
flow of the program.
ROM correction can be used in a maximum of four locations within the internal ROM (program).
Caution Note that ROM correction cannot perform emulation in the in-circuit emulator (IE-784000-R, IE-
784000-R-EM).
In more detail, the command addresses that require repair from the inactive memory externally connected to a
microcontroller by a user program and the repair command codes are loaded into the peripheral RAM.
The above addresses and the internal ROM access addresses are compared by the comparator built into the micro
computer during execution of internal ROM programs (during command fetch), and internal ROM’s output data is then
converted to call command (CALLT) codes and output when a match is determined.
When the CALLT command codes are changed to valid commands by the CPU and executed, the CALLT table
is referenced, and the process routine and other peripheral RAM are branched. At this point, a CALLT table is prepared
for each repair address for referencing purposes. Four repair address can be set for the
µ
PD784225.
Match-ups with address pointer 0: CALLT table (0078H)
Conversion command code: FCH
Match-ups with address pointer 1: CALLT table (007AH)
Conversion command code: FDH
Match-ups with address pointer 2: CALLT table (007CH)
Conversion command code: FEH
Match-ups with address pointer 3: CALLT table (007EH)
Conversion command code: FFH
Caution As it is necessary to reserve four locations for the CALLT tables when the ROM correction
function is used (0078H, 007AH, 007CH, 007EH), ensure that these are not used for other
applications. However, the CALLT tables can be used if the ROM correction function is not being
used.
The differences between 78K/IV ROM correction and 78K/0 ROM correction are shown in Table 26-1.
