DescriptionQ406
Q406DVIntelASIC interview problem
Constrain a bounded packet split
TechniquesDVSystemVerilogUVMConstrainsplit
DifficultyMedium
TopicSystemVerilog & UVM
LanguageSystemVerilog
Requirements4 checkpoints
01
Problem
A transfer is divided into a fixed number of random packet lengths while leaving enough bytes for every remaining packet. Write a SystemVerilog constraint class for the packet lengths.
Class declarationSystemVerilog
class packet_split;
rand int unsigned len[8];
int unsigned total, count, min_len, max_len;
constraint legal_c;
endclassExample input and output
Use this case to check your interpretationInput
total=12, count=3, min_len=4, and max_len=4.Output
The only legal randomized array is {4,4,4,0,0,0,0,0}.Explanation
Three active entries must each equal four and sum to twelve, while every inactive entry is constrained to zero.
02
Requirements (4)
- randomize() is legal only when 1<=count<=8, max_len<=65535, min_len<=max_len, total<=65535, and count*min_len<=total<=count*max_len.
- For every active i<count, require min_len<=len[i]<=max_len and the sum of active lengths to equal total.
- Set every inactive len[i] for i>=count to zero.
- Add ordering or solve-before constraints only if needed for solver clarity; do not use post_randomize to repair an illegal result.