The F256 Jr is a 65C02 based system which includes the following features:

512K of Shared System & Video RAM (256k on the Rev A)
256k of flash
256k of cartridge ram/rom
TinyVICKY Graphic Chip (CFP9599)
1x Simple 3-wire UART (RS-232 Port, Feather ESP8266 wifi, or Feather MIDI)
2x Atari Style Joystick Ports, 1x DB9 to interface to NES/SNES Adapter (For Rev B Only)
1x IEC Port to interface to Commodore Drives
2x Internal PSG Sound Chip
2x External SID Sockets

Aside from its sound chip, it's very similar to the F256k.

Hardware Setup

The PS2 mouse port may be wired to the keyboard port and then split out using a laptop PS2 splitter.

If your Jr. is booting to a blank screen with a dark purple border, be sure that the RAM/FLASH jumper is set to FLASH.

Kernels

There are two kernels for the Jr.

There is a simple example DOS shell for the MicroKernel in the kernel/dos project: https://github.com/ghackwrench/F256_Jr_Kernel_DOS
It may also be found in the SuperBASIC project under source/releases: https://github.com/paulscottrobson/superbasic
SuperBASIC's install scripts install it by default. To access it, type "DOS" from BASIC.
To return to SuperBASIC, type "basic <enter>" from DOS.
To auto-boot into DOS instead of BASIC, install DOS in flash slot 1, and BASIC in slots 2,3,4.