# PROJECT 4: STACK

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DIGITAL LOGIC SYSTEMS
PROJECT 4: STACK

The synchronous circuit Stack(m, n) implements a LIFO (Last in First
out) stack. The circuit is able to store at most m binary words, each word is
n-bits wide.
Specifications
Inputs: push, pop, CLK ∈ {0, 1}; Din ∈ {0, 1}
n
Outputs: Dout ∈ {0, 1}
n
; full, empty ∈ {0, 1}
Functionality:
(1) If push(t) = ‘1’ then Din[n − 1 : 0](t) will be stored at the head
of the stack: Dout[n − 1 : 0](t + 1) = Din[n − 1 : 0](t). All
the previously stored items are pushed downwards. If the stack
already holds m words, then the push won’t take place.
(2) If pop(t) = ‘1’ then the head item is removed from the stack,
and the next item takes its place.
(3) full = ‘1’ iff the stack holds m items.
(4) empty = ‘1’ iff the stack holds 0 items.
(5) Dout[n − 1 : 0] outputs the head-word of the stack. If the stack
is empty, then Dout[n − 1 : 0] = 0n
is outputted.
(6) Assume that the inputs push and pop may not be high simultaneously.
CLK
pop
push
Din Dout
3 3
full
empty
010
111
000
101
110
000
000
000
Figure 1. A schematic example of Stack(8, 3). The gray
cells represent the occupied cells. In this example, the values
that have been pushed to the stack are: ‘110’, ‘101’, ‘000’,
‘111’ and the last one was ‘010’. The current head-word is
‘010’. If pop = 1 is fed for one clock cycle, then ‘010’ will be
thrown out and ‘111’ will become the head-word.
1
DLS: PROJECT 4 2
(1) Complete the circuit Stack from the template template stack.circ
to implement Stack(3, 3).
(2) The clock period of a synchronous circuit is defined according to the
longest path between two flip flops (assume that the IO ports are
also coming to/from flip-flops). Make sure that the clock period is
constant (O(1)) and doesn’t depend on n.
Submission Instructions
(1) Submit a single Logisim (“.circ”) file. No prints/screenshots. This
file must be named ID1 ID2 stack.circ with ID1 and ID2 replaced
by each partner’s 9 digit ID number.
(2) Use the provided template stack.circ file as a template, and implement your designs in the circuit named Stack. Do not move or
modify the input/output ports, the “blackbox” layout, and the names
of the circuits!
(3) Only one of the students in a pair needs to upload the submission.
Do not upload the same work twice!
(4) You may not use gates with fan-in larger than 2. Exception is for
Logisim’s MUX2:1, with a fan-in of 3, however is allowed to be used.
(5) You are allowed to use the data-bits attribute of the gates to perform
bitwise operations.
(6) You are allowed to use “Arithmetic” library of Logisim (Adder,Shifter
circuits). as well as the “Plexers” library.
(7) From the “Memory” Library you are allowed to use the “D Flip-Flop”
element as well as the “Register” element. The Register is a parallel
load register we studied, which has a clock enabled input.

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