Question: Adam Dunkels is the author of which operating system useful for memory-constrained systems?

Context paragraphs:
RIOT (operating system): RIOT is a small operating system for networked, memory-constrained systems with a focus on low-power wireless Internet of Things (IoT) devices.  It is open-source software, released under the GNU Lesser General Public License (LGPL).  Due to this unclonable license and its large independent community RIOT is often referred to as the Linux of the Internet of Things.

Embedded operating system: An embedded operating system is an operating system for embedded computer systems.  This type of operating system is typically designed to be resource-efficient and reliable.  Resource efficiency comes at the cost of losing some functionality or granularity that larger computer operating systems provide, including functions which may not be used by the specialized applications they run.  Depending on the method used for multitasking, this type of OS is frequently considered to be a real-time operating system, or "RTOS".

OS/360 Object File Format: The OS/360 Object File Format is the standard object module file format for the IBM DOS/360, OS/360 and VM/370, Univac VS/9, and Fujitsu BS2000 mainframe operating systems.  In the 1990s, the format was given an extension with the XSD-type record for the MVS Operating System to support longer module names in the C Programming Language.  This format is still in use by the z/VSE operating system (the follow-on to the DOS/360 Operating System).  In contrast, it has been superseded by the GOFF file format on the MVS Operating System (the follow-on to the OS/360 Operating System) and on the z/VM Operating System (the follow-on to the VM/370 Operating System).  Since the MVS and z/VM loaders will still handle this older format, some compilers have chosen to continue to produce this format instead of the newer GOFF format.

Contiki: Contiki is an operating system for networked, memory-constrained systems with a focus on low-power wireless Internet of Things devices.  Extant uses for Contiki include systems for street lighting, sound monitoring for smart cities, radiation monitoring, and alarms.  It is open-source software released under a BSD license.

MS-DOS: MS-DOS ( ; acronym for "Microsoft Disk Operating System") is a discontinued operating system for x86-based personal computers mostly developed by Microsoft.  Collectively, MS-DOS, its rebranding as IBM PC DOS, and some operating systems attempting to be compatible with MS-DOS, are sometimes referred to as "DOS" (which is also the generic acronym for disk operating system).  MS-DOS was the main operating system for IBM PC compatible personal computers during the 1980s and the early 1990s, when it was gradually superseded by operating systems offering a graphical user interface (GUI), in various generations of the graphical Microsoft Windows operating system.

Security-focused operating system: This is a list of operating systems with a sharp security focus.  Here, "security-focused" means that the project is devoted to increasing the security as a major goal.  As such, something may be secure without being "security-focused."  For example, almost all of the operating systems mentioned here are faced with security bug fixes in their lifetime; however, they all strive consistently to approach all generic security flaws inherent in their design with new ideas in an attempt to create a secure computing environment.  Security-focused does not mean security-evaluated operating system, which refers to operating systems that have achieved certification from an external security-auditing organization.  An operating system that provides sufficient support for multilevel security and evidence of correctness to meet a particular set of government requirements is called a "trusted operating system".

LiteOS: LiteOS is a real-time operating system (RTOS) from University of Illinois for use in sensor networks. LiteOS is a Unix-like operating system that fits on memory-constrained sensor nodes.  This operating system allows users to operate wireless sensor networks like operating Unix, which is easier for people with adequate Unix background.  LiteOS provides a familiar programming environment based on Unix, threads, and C.  It follows a hybrid programming model that allows both event-driven and thread-driven programming.  LiteOS is open source, written in C and runs on the Atmel AVR based MicaZ and IRIS sensor networking platform.

Adam Dunkels: Adam Dunkels is a Swedish entrepreneur, programmer and founder of Thingsquare.  His father was Andrejs Dunkels, professor in Mathematics.  His mother was Kerstin Vännman also professor.His work is mainly focused on networking technology and distributed communication for small embedded devices and wireless sensor networks on the Internet.  Dunkels is best known to the embedded community as the author of the uIP (micro-IP) and lwIP TCP/IP protocol stacks.  He is also the creator of protothreads and author of the Contiki operating system.  The "MIT Technology Review" placed him on the TR35 list of world's top 35 innovators under 35, in 2009.

UIP (micro IP): The uIP is an open source implementation of the TCP/IP network protocol stack intended for use with tiny 8- and 16-bit microcontrollers.  It was initially developed by Adam Dunkels of the "Networked Embedded Systems" group at the Swedish Institute of Computer Science, licensed under a BSD style license, and further developed by a wide group of developers.

LwIP: lwIP (lightweight IP) is a widely used open source TCP/IP stack designed for embedded systems. lwIP was originally developed by Adam Dunkels at the Swedish Institute of Computer Science and is now developed and maintained by a worldwide network of developers.

Supporting paragraphs: Adam Dunkels Contiki
Answer: Contiki