UPD784938 Datasheet, PDF (76/733 Pages) NEC – 16-Bit Single-Chip Microcontrollers

CHAPTER 3 CPU ARCHITECTURE
3.3.2 CALLT instruction table area
The 1-byte call instruction (CALLT) subroutine entry addresses can be stored in the 64-byte area from 00040H to 0007FH.
The CALLT instruction references this table, and branches to a base area address written in the table as a subroutine. As
the CALLT instruction is one byte in length, use of the CALLT instruction for subroutine calls written frequently throughout the
program enables the program object size to be reduced. The table can contain up to 32 subroutine entry addresses, and
therefore it is recommended that they be recorded in order of frequency.
If this area is not used as the CALLT instruction table, it can be used as ordinary program memory or data memory.
3.3.3 CALLF instruction entry area
A subroutine call can be made directly to the area from 00800H to 00FFFH with the 2-byte call instruction (CALLF).
As the CALLF instruction is a two-byte call instruction, it enables the object size to be reduced compared with use of the direct
subroutine call CALL instruction (3 or 4 bytes).
Writing subroutines directly in this area is an effective means of exploiting the high-speed capability of the device.
If you wish to reduce the object size, writing an unconditional branch (BR) instruction in this area and locating the subroutine
itself outside this area will result in a reduced object size for subroutines that are called from five or more points. In this case,
only the 4 bytes of the BR instruction are occupied in the CALLF entry area, enabling the object size to be reduced in a large
number of subroutines.
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Preliminary User’s Manual U13987EJ1V0UM00