Uploading to ESP32 is explained in esp32ecp5 project.

Upload "osd.py" and "ld_nes.py" to ESP32.

Compile NES bitstream "nes.bit" with ESP32 support. Also USB support is recommended if you have compatible joystick (darfon, saitek, xbox360).

Option 1: bitstream in SPI FLASH:

ujprog -j flash nes.bit

in "nes.py" you need to comment lines for ecp5 loading.

Option 2: bitstream in ESP32 FLASH:

gzip -9 nes.bit

upload "ecp5.py", "nes.bit.gz" to root directory of ESP32.

Download few ROMs from some site like wowroms, unzip ROM file and upload it to SD card, it can be done using "uftpd.py", SD card reader is not required.

Start ESP32 NES OSD loader:

screen /dev/ttyUSB0 115200
>>> import nes

It will upload NES bitstream "nes.bit.gz" to ECP5 and register interrupt handler, it should respond to those keypresses on ULX3S board:

UP+DOWN+LEFT+RIGHT (pressed together): directory list window open/close
UP or DOWN: move cursor in the directory list
RIGHT: enter subdirectory or load ROM file
LEFT: back one subdirectory level