The document discusses various data types in System Verilog, contrasting them with Verilog-95 types. It outlines the distinctions between 4-state and 2-state data types, array types including fixed and dynamic arrays, and user-defined types, including enumerated types and their applications. Additionally, it highlights best practices for using these data types effectively in simulation and verification contexts.

1. System Verilog Data Types
(Chapter 2 )

2. Data Types in Verilog-95
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‘Reg’ and ‘net’
• They all hold 4-state values (0, 1, Z and X)
• They can be single bit, multibit (eg: vectors), signed 32 bit(eg:
integer), unsigned 64 bit (eg: time), signed 64 bit floating point
(eg: real).
• All are static => retain values throughout simulation.
• Can’t hold local variables

3. New Data Types in System verilog
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4. ‘Logic’ data type
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• One data type for both combinational and sequential.. Avoid headache of deciding
between ‘reg’ or ‘net’.
• Can be driven by continuous assignments, gates, modules and for storing variables.
• Limitation
• Can’t be driven by multiple drivers (eg: modeling a bus). Need to use ‘wire’ for
multiple drivers

5. ‘2 state’ data types
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• In Verilog, all data types are 4-states.
• System verilog introduces ‘two-state’ data types (hold only ‘0’ or ‘1’).
• We use them where ‘x’ and ‘z’ not needed eg: test benches, loop variables,
• Make the simulators more efficient and also reduces memory storage requirements.
Ans: No.. One is 2 state and other is 4 state. Also, one is signed and
other is unsigned.. Hence range of data stored is different.
*Note
1) System verilog stores each element as a long-word (32 bit). Longint is stored in 2 long
words. Four state type like logic is also stored as 2-long words (64 bits)
2) ‘int’ is a 2 state data type while ‘integer’ is a 4 state data type.
Ques: Can we use ‘byte’ and logic[7:0] interchangably?

6. ‘2 state’ data types: warning
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• Be careful while connecting 2 state variables to outputs of DUT..
• If pin drives ‘X’ or ‘Z’, it is converted to two state values (either 0 or 1).
• You will be working with incorrect data..
• Hence if you have this connection, use ‘$isunknown’ command to check if any
unknown (x or z for 2 state variable) output is being got..

7. Arrays
Fixed Size Arrays
&
Dynamic Arrays
(Verilog has only fixed size arrays)
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8. Fixed Size Arrays
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• Lower bound is assumed to be zero if not mentioned (just like C ). In Verilog we need
to explicitly provide upper and lower bounds.
Will this work in Verilog?
Multi-dimensional arrays

9. Initializing Arrays
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10. Array Operations (for, foreach)
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11. Array Operations (multi-dimensional)
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Note the syntax :md[I,j]

12. Copy and Compare
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Note: we can’t perform
aggregate arithmetic
operations like addition etc
on arrays.
Need to use loops

13. Packed and Unpacked arrays
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Each array is only 8 bits long, but by default it is
stored in a long word (32 bits)
Unpacked Array

14. Packed and Unpacked arrays
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• For some data types, we may want to access entire value or a part of it.
• Eg: 32 bit register, we may want to access as a 32 bit value or as 4 separate
bytes.
• It is treated as both an array and a single value
• Stored in contiguous locations, no unused space is left.
Packed Array

15. Mixing Packed and Unpacked arrays
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• For some data types, we may want to access entire value or a part of it.
• Eg: 32 bit register, we may want to access as a 32 bit value or as 4 separate
bytes.
• It is treated as both an array and a single value
• Stored in contiguous locations, no unused space is left.

16. Dynamic Arrays
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What if we don’t know the size of the array till run-time.
If we allocate too many => waste of memory
Create 20 locations
and copy old five.

17. Copying between Fixed and Dynamic Arrays
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Fixed array can be copied to a dynamic array as long as both have the same base type
(eg : int, float).
Dynamic array can be copied to a fixed array as long as they have same number of
elements and the type matches.

18. Queues
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Indexing starts from 0
Note: when a queue is created, some amount of space is
allocated so that new elements can be added.. No need to
call ‘new’ function repeatedly

19. Associative Arrays
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What if we want to create very large arrays (Gb memory modelling for processors).
We may be accessing only a few thousand locations from random points (executable code
from particular location, data from somewhere else)…
Hence.. Even dynamic arrays initializing is waste of memory locations.
Associative memory: can address very large address space but allocates memory only when
we write in to it. They can be indexed using any of the supported data types.
Only these locations
are allocated, rest
are free
Similar to pointers…. Use *
“Tree” data structure is used to store and access the array.

20. Associative Arrays
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Foreach (variable[iterator])
The iterator can be anything, not
needed to be defined before.
1) Study about “addressing
using strings”.
‘first’ and ‘next’ are functions that
modify the index and return a ‘0’ or ‘1’
depending on existence of data in the
position

21. Arrays Methods
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These can be used on any unpacked array types (fixed, dynamic, queue, associative)
Self study:
How to choose storage type based on need (flexibility, speed, memory
usage and sorting requirements)
• Array Reduction Methods
• Sum, product, or, xor
• Array locator methods
• Find, find_index, find_first, find_last, min, max, unique

22. User Defined Types
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Very useful data type: unsigned, 2 state, 32 bit integer. (most values in testbenches are positive..
Hence useful in verification)
Not really a new type, just a macro for variable size

23. User Defined Structures
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Creates a single pixel
Creates a new type for creating
multiple pixels
Packed structure.. To save space.. Not use 3 long words ..

24. Enumerated Types
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We create a new
enum data type
and then define
variables based
on that.
Start from ‘0’ by
default.
Color is assigned red/blue/green
Default values are overridden.
Init = 0, decode =2, idle = 3

25. Enumerated Types
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Remember :
Enumerated variables are stored as int (2-state).
Int are by default initialized to 0.
Find out if there is any fault in the following :
‘position’ is initialized to 0. but
zero is not a legal vaue of type
ordinal_e

26. Enumerated Types
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27. Strings
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Remember :
String indexes from 0 -> N-1
No null character at end.
Uses dynamic memory. so no worries about running out of space.

28. Tata.. bye bye.. ‘data types’
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