There are two methods of flash programming that are widely used during development:

1.

The ML674K/5K MCUs, have a boot-ROM built into them. When the external BSEL[1:0] pins (switches on the CPU-board) are configured to boot from this boot ROM, the following sequence occurs:   a. The boot ROM initializes the chip and the UART   b. The boot ROM synchs the UART with the baud-rate from the development host UART   c. The boot ROM can download a flash utility (provided by Oki or customer designed) to internal RAM of the MCU and executes it after the internal RAM is remapped to address 0x00   d. The Flash Utility will download code through the UART and write the internal flash of the MCU. The flash utility can also be configured to do other things such as write external flash or use a different peripheral for download

2.

The second method is to use JTAG to download a flash utility to the board and execute it. The data to be written to the flash can also be downloaded through the JTAG. This method is much faster and simpler to do during development.

In addition, many debuggers provided by Oki partners have the feature to program the internal flash of Oki Advantage Microcontrollers on the fly. Although it's not always obvious, these debuggers are actually using method 2 from above.