16
μ
PD750064, 750066, 750068, 750064(A), 750066(A), 750068(A)
Data Sheet U10165EJ2V0DS00
4. SWITCHING FUNCTION BETWEEN Mk I MODE AND Mk II MODE
4.1 Differences between Mk I Mode and Mk II Mode
The CPU of the
μ
PD750068 has the following two modes: Mk I and Mk II, either of which can be selected.
The mode can be switched by the bit 3 of the stack bank select register (SBS).
Mk I mode:
Upward compatible with
μ
PD75068. Can be used in the 75XL CPU with a ROM capacity
of up to 16 Kbytes.
Incompatible with
μ
PD75068. Can be used in all the 75XL CPUs including those products
whose ROM capacity is more than 16 Kbytes.
Mk II mode:
Table 4-1. Differences between Mk I Mode and Mk II Mode
Mk I Mode
Mk II Mode
Number of stack bytes
for subroutine instructions
2 bytes
3 bytes
BRA !addr1 instruction
CALLA !addr1 instruction
Not available
Available
CALL !addr instruction
3 machine cycles
4 machine cycles
CALLF !faddr instruction
2 machine cycles
3 machine cycles
Caution
The Mk II mode supports a program area exceeding 16 Kbytes for the 75X and
75XL Series. Therefore, this mode is effective for enhancing software compatibility
with products exceeding 16 Kbytes.
When the Mk II mode is selected, the number of stack bytes used during
execution of subroutine call instructions increases by one byte per stack
compared to the Mk I mode. When the CALL !addr and CALLF !faddr instructions
are used, the machine cycle becomes longer by one machine cycle. Therefore,
use the Mk I mode if the RAM efficiency and processing performance are more
important than software compatibility.