# An 8 Kbyte ROM with an active low Chip Select input$\left( {\overline {{\rm{CS}}} } \right)$ is to be used in an 8085 microprocessor-based system. T

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An 8 Kbyte ROM with an active low Chip Select input$\left( {\overline {{\rm{CS}}} } \right)$ is to be used in an 8085 microprocessor-based system. The ROM should occupy the address range 1000H to 2FFFH. The address lines are designated as A15 to A0, where A15 is the most significant address bit. Which one of the following logic expressions will generate the correct $\overline {{\rm{CS}}}$ signal for this ROM?
1. A15 + A14 + (A 13 . A12 + A̅13 . A̅ 12)
2. A15 . A14 . (A13 + A12)
3. A̅15 . A̅ 14 . (A 13 . A̅12
4. A̅15 + A̅14 + (A13 . A12)

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Correct Answer - Option 1 : A15 + A14 + (A 13 . A12 + A̅13 . A̅ 12)

Given address range: 1000 H to 2FFF

∴ Total No. of address lines = 13 i.e.  $\begin{array}{*{20}{c}} {2{\rm{FFF}}}\\ {\underline {1000} }\\ {1{\rm{FFF}}} \end{array}$

(1FFF)H = (0001 1111 1111 1111)2 = 13 address lines

i.e. $\begin{array}{*{20}{c}} {{{\rm{A}}_{15}}}&{{{\rm{A}}_{14}}}&{{{\rm{A}}_{13}}}&{{{\rm{A}}_{12}}}&{{{\rm{A}}_{11}}}&{{{\rm{A}}_{10}}}& \cdots &{{{\rm{A}}_2}}&{{{\rm{A}}_1}}&{{{\rm{A}}_0}}&{}\\ 0&0&0&1&0&0&{}&0&0&0&{ = {{\left( {1000} \right)}_{\rm{H}}}}\\ {}&{}& \vdots &{}&{}& \vdots &{}&{}&{}&{}&{}\\ 0&0&1&0&1&1&{}&1&1&1&{ = {{\left( {2{\rm{FFF}}} \right)}_{\rm{H}}}} \end{array}$

i.e. $\begin{array}{*{20}{c}} {{{\rm{A}}_{15}}}&{{{\rm{A}}_{14}}}&{{{\rm{A}}_{13}}}&{{{\rm{A}}_{12}}}\\ 0&0&0&1\\ 0&0&1&0 \end{array}$

To provide $\overline {{\rm{CS}}}$ as low, the condition is

A15 = A14 = 0 and A13 = A12 = (01) or (10)

i.e. A15 = A14 = 0 and A13 , A12 shouldn't be (11) , (00)

Thus it is A15 + A14 + (A 13 . A12 + A̅13 . A̅ 12)

Hence option (1) is correct