This paper provides a detailed study to design and simulate all-optical half adder using terahertz optical asymmetric demultiplexer (TOAD). The effect of the semiconductor optical amplifier parameters on the operation of TOAD is studied. Through the use of the designed all-optical half adder unit, design of all-optical full adder, S-R flip flop, 1 bit random access memory (RAM) and 4 9 4 RAM are proposed. The contrast ratio, extinction ratio and amplitude modulation values are analyzed for all-optical half adder. The performance of all the proposed designs is investigated at 10 Gbps by using OptiSystem simulation software.Keywords Terahertz optical asymmetric demultiplexer (TOAD) Á Random access memory (RAM) Á Semiconductor optical amplifier (SOA) Á All-optical switching devices