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60 User’s Manual U10676EJ3V0UM
CHAPTER 4 INTERNAL CPU FUNCTION
4.1 Function to Select MkI and MkII Modes
4.1.1 Difference between MkI and MkII modes
The CPU of the
µ
PD754244 has two modes to be selected: MkI and MkII. These modes can be selected by using
bit 3 of the stack bank select register (SBS).
• MkI mode: In this mode, the
µ
PD754144 is upwardly-compatible with the 75X Series.
This mode can be used with the CPU in the 75XL Series having a ROM capacity of up to 16 KB.
• MkII mode: In this mode, the
µ
PD754144 is not compatible with the 75X Series.
This mode can be used with all the CPUs in the 75XL Series, including the models having a ROM
capacity of 16 KB or higher.
Table 4-1. Differences Between MkI and MkII Modes
MkI Mode MkII Mode
Number of stack bytes of 2 bytes 3 bytes
subroutine instruction
BRA !addr1 instruction Not provided Provided
CALLA !addr1 instruction
CALL !addr instruction 3 machine cycles 4 machine cycles
CALLF !faddr instruction 2 machine cycles 3 machine cycles
Caution The MkII mode supports a program area exceeding 16 KB for the 75X and 75XL Series. This mode
enhances software compatibility of the
µ
PD754244 with a product with a program area of more
than 16 KB.
When the MkII mode is selected, The number of stack bytes increases by one byte per stack, as
compared with the MkI mode, when the subroutine call instruction is executed. When the CALL
!addr or CALLF !faddr instruction is used, the machine cycle is extended by 1 cycle. To
emphasize the use efficiency of the RAM or processing capability more than software compat-
ibility, therefore, use the MkI mode.