Design a single board microcomputer system which incorporates the Intel $8086(5$MHz$)$ microprocessor and contains $250$KByte of memory starting at address $0$H$.$ This $250$KByte of memory should be divided into $64$KB of SRAM at bottom, $128$KB of DRAM at middle and $64$KB of EPROM at the top. The available parts are $32$ KB of EPROM, SRAM and DRAM chips, OR gates and $3$ to $8$ decoder. How much time is allowed by the microprocessor for memory access?