4-2
MSM66573 Family User's Manual
Chapter 4 Memory Control Functions
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 4 bits indicate the end address of the
ROM window. (Bits 4 and 5 of the upper 4 bits must be written as "1"s.) 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.
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 4 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
4321
0
ROMWIN
At reset
(undefined)
Address: 000B [H]
R/W access: R/W
(W is only performed once after reset)
"1"
Bits 4 and 5 must be set to "1".
Figure 4-1 ROMWIN Configuration