DS89C420_05 Datasheet, PDF (20/47 Pages) Maxim Integrated Products – Ultra-High-Speed Microcontroller

Figure 6. Memory Map
DS89C420 Ultra-High-Speed Microcontroller
FFFF
FFFF
INTERNAL
REGISTERS
128 Bytes SFR
FF
128 Bytes
Indirect
Addressing
80
7F
2F
20 ^{Bit Addressable}
1F _{Bank 3}
Bank 2
Bank 1
Bank 0
00
03FF
INTERNAL
MEMORY
1K x 8
SRAM
Data OR
prog mem
addr from
0000 ^{400 - 7FF}
3FFF
8K x 8
Flash
Memory
(Program)
2000
1FFF
8K x 8
Flash
Memory
(Program)
0000
External
Program
Memory
4000
0000
External
Data
Memory
03FF
0000
The reset default condition is a maximum on-chip program-memory address of 16kB. When accessing external
program memory, the first 16kB would be inaccessible. To select a smaller effective program memory size,
software must alter bits RMS2–RMS0. Altering these bits requires a timed access procedure as explained later.
Care should be taken so that changing the ROMSIZE register does not corrupt program execution. For example,
assume that a DS89C420 is executing instructions from internal program memory near the 12kB boundary
(~3000h) and that the ROMSIZE register is currently configured for a 16kB internal program space. If software
reconfigures the ROMSIZE register to 4kB (0000h–0FFFh) in the current state, the device immediately jumps to
external program execution because program code from 4kB to 16kB (1000h–3FFFh) is no longer located on-chip.
This could result in code misalignment and execution of an invalid instruction. The recommended method is to
modify the ROMSIZE register from a location in memory that is internal (or external) both before and after the
operation. In the above example, the instruction that modifies the ROMSIZE register should be located below the
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