
ML66517 Family User’s Manual
Chapter 4
Memory Control Functions
4 – 2
4.3
ROM Window Function
The ROM window function reads the contents of the program memory space specified by the ROM window
register (ROMWIN), located in the SFR area, by using the same address in the data memory space as a window.
In other words, when the ROM window function is enabled and an instruction that accesses (reads) the data
memory space is executed, instead of accessing (reading) data in the data memory space, data will be accessed
(read) at the same addresses in the segment that is specified by TSR in the program memory space.
Compared to the number of instruction cycles to be required to access normal data memory, accessing the ROM
window once requires additional 3 cycles for a byte instruction and additional 6 cycles for a word instruction.
[Note]
If the ROM window function is enabled and a write instruction is executed, that result will not be guaranteed.
However, in this case additional cycles will not be added.
ROM Window Register (ROMWIN)
The ROM window register (ROMWIN) is an 8-bit register. The lower 4 bits indicate the start address of the
ROM window and the upper 2 bits indicate the end address of the ROM window. When 64KB of the
program memory space is represented in hexadecimal number (HEX), each of above 4-bit registers
specifies the upper 1 digit of 4 digits. If the value of the lower 4 bits is all zeros, the ROM window function
will not operate.
Figure 4-1 shows the configuration of ROMWIN.
Figure 4-1
ROMWIN Configuration
These 4 bits specify the start
address of the ROM window.
(The upper 1 digit of 4 digits when
the 64KB memory space is represented
in hexadecimal format (HEX).)
These 2 bits specify the end
address of the ROM window.
(The upper 1 digit of 4 digits when
the 64KB memory space is represented
in hexadecimal format (HEX).)
7
6
5
4
3
2
1
0
1
0
ROMWIN
At reset
Address: 000B [H]
R/W access: R/W
(W is only performed once after reset)
“
” indicates a nonexistent bit.
When read, its value will be “1”.