来自: 这里
If you want more control over randomizing real numbers in terms of sign, exponential and mantissa, use $bitstoreal() as shown in example below. For positive numbers, use sgn = 0 etc.
EXAMPLE:
module Tb();
reg sgn;
reg [10:0] exp; reg [51:0] man;
real r_num;
initial
begin
repeat(5)
begin
sgn = $random;
exp = $random;
man = $random;
r_num = $bitstoreal({sgn,exp,man});
$display("r_num = %e",r_num);
end
end
endmodule
RESULTS:
r_num = 3.649952e+193
r_num = -1.414950e-73
r_num = -3.910319e-149
r_num = -4.280878e-196
r_num = -4.327791e+273
Sometimes it is required to generate random numbers without repetition. The random numbers should be unique. For example, to generate 10 random numbers b/w 0 to 9 without repetition, the following logic can be used.
EXAMPLE:
module Tb();
integer num,i,j,index;
integer arr[9:0];
reg ind[9:0];
reg got;
initial
begin
index=0;
for(i=0;i<10;i=i+1)
begin
arr[i] = i;
ind[i] = 1;
end
for(j = 0;j<10 ;j=j+1)
begin
got = 0;
while(got == 0)
begin
index = { $random() } % 10;
if(ind[index] == 1)
begin
ind[index] = 0;
got = 1;
num = arr[index];
end
end
$write("| num=%2d |",num);
end
end
endmodule
RESULT:
| num= 8 || num= 7 || num= 5 || num= 2 || num= 1 || num= 9 || num= 6 || num= 4 || num= 0 || num= 3 |
Random number system function has a argument called seed. The seed parameter controls the numbers that $random returns such that different seeds generate different random streams. The seed parameter shall be either a reg, an integer, or a time variable. The seed value should be assigned to this variable prior to calling $random. For each system function, the seed parameter is an in-out parameter; that is, a value is passed to the function
and a different value is returned.
EXAMPLE:
module Tb();
integer num,seed,i,j;
initial
begin
for(j = 0;j<4 ;j=j+1)
begin
seed = j;
$display(" seed is %d",seed);
for(i = 0;i < 10; i=i+1)
begin
num = { $random(seed) } % 10;
$write("| num=%2d |",num);
end
$display(" ");
end
end
endmodule
RESULT:
seed is 0
| num= 8 || num= 7 || num= 7 || num= 7 || num= 7 || num= 7 || num= 5 || num= 2 || num= 1 || num= 9 |
seed is 1
| num= 8 || num= 8 || num= 2 || num= 2 || num= 6 || num= 3 || num= 8 || num= 5 || num= 5 || num= 5 |
seed is 2
| num= 8 || num= 1 || num= 0 || num= 5 || num= 0 || num= 8 || num= 6 || num= 7 || num= 1 || num= 6 |
seed is 3
| num= 8 || num= 2 || num= 2 || num= 3 || num= 8 || num= 6 || num= 1 || num= 4 || num= 3 || num= 9 |
The $random function has its own implicit variable as seed when the used is not giving explicitly giving seed. The following example shows that seed = 0 and implicit seed are having same sequence. It means that the implicitly taken seed is also 0.
EXAMPLE:
module Tb();
integer num,seed,i,j;
initial
begin
seed = 0;
for(j = 0;j<2 ;j=j+1)
begin
if(j ==0)
$display(" seed is %d",seed);
else
$display(" No seed is given ");
for(i = 0;i < 10; i=i+1)
begin
if( j == 0)
num = { $random(seed) } % 10;
else
num = { $random() } % 10;
$write("| num=%2d |",num);
end
$display(" ");
end
end
endmodule
RESULT:
seed is 0
| num= 8 || num= 7 || num= 7 || num= 7 || num= 7 || num= 7 || num= 5 || num= 2 || num= 1 || num= 9 |
No seed is given
| num= 8 || num= 7 || num= 7 || num= 7 || num= 7 || num= 7 || num= 5 || num= 2 || num= 1 || num= 9 |
The system functions shall always return the same value given the same seed. This facilitates debugging by making the operation of the system repeatable. The argument for the seed parameter should be an integer variable that is initialized by the user and only updated by the system function. This ensures the desired distribution is achieved.
EXAMPLE:
module Tb();
integer num,seed,i,j;
initial
begin
for(j = 0;j<4 ;j=j+1)
begin
seed = 2;
$display(" seed is %d",seed);
for(i = 0;i < 10; i=i+1)
begin
num = { $random(seed) } % 10;
$write("| num=%2d |",num);
end
$display(" ");
end
end
endmodule
RESULT:
seed is 2
| num= 8 || num= 1 || num= 0 || num= 5 || num= 0 || num= 8 || num= 6 || num= 7 || num= 1 || num= 6 |
seed is 2
| num= 8 || num= 1 || num= 0 || num= 5 || num= 0 || num= 8 || num= 6 || num= 7 || num= 1 || num= 6 |
seed is 2
| num= 8 || num= 1 || num= 0 || num= 5 || num= 0 || num= 8 || num= 6 || num= 7 || num= 1 || num= 6 |
seed is 2
| num= 8 || num= 1 || num= 0 || num= 5 || num= 0 || num= 8 || num= 6 || num= 7 || num= 1 || num= 6 |
Seed is inout port. Random number system function returns a random number and also returns a random number to seed inout argument also. The results of the following example demonstrates how the seed value is getting changed.
EXAMPLE:
module Tb();
integer num,seed,i,j;
initial
begin
seed = 0;
for(j = 0;j<10 ;j=j+1)
begin
num = { $random(seed) } % 10;
$write("| num=%2d |",num);
$display(" seed is %d ",seed);
end
end
endmodule
RESULT:
| num= 8 | seed is -1844104698
| num= 7 | seed is 1082744015
| num= 7 | seed is 75814084
| num= 7 | seed is 837833973
| num= 7 | seed is -2034665166
| num= 7 | seed is -958425333
| num= 5 | seed is 851608272
| num= 2 | seed is 154620049
| num= 1 | seed is -2131500770
| num= 9 | seed is -2032678137
From the above results we can make a table of seed values and return values of $random. If a seed is taken from the table, then rest of the sequence has to follow sequence in table.
Table is as falows for initial seed 0;
| num= 8 | seed is -1844104698
| num= 7 | seed is 1082744015
| num= 7 | seed is 75814084
| num= 7 | seed is 837833973
| num= 7 | seed is -2034665166
| num= 7 | seed is -958425333
| num= 5 | seed is 851608272
| num= 2 | seed is 154620049
| num= 1 | seed is -2131500770
| num= 9 | seed is -2032678137
.
.
.
.
.
table goes on........
In the following example, the seed is 837833973, which is the 4 th seed from the above table.
EXAMPLE:
module Tb();
integer num,seed,i,j;
initial
begin
seed = 837833973;
for(j = 0;j<10 ;j=j+1)
begin
num = { $random(seed) } % 10;
$write("| num=%2d |",num);
$display(" seed is %d ",seed);
end
end
endmodule
RESULTS:
| num= 7 | seed is -2034665166
| num= 7 | seed is -958425333
| num= 5 | seed is 851608272
| num= 2 | seed is 154620049
| num= 1 | seed is -2131500770
| num= 9 | seed is -2032678137
| num= 8 | seed is -1155272804
| num= 7 | seed is -1634874387
| num= 9 | seed is -153856566
| num= 2 | seed is -970066749
From the above example we can come to conclusion that $random is not giving a random number. It is randomizing seed and returning corresponding number for that seed.
Total possible seed values are 4294967295. Is it possible for $random to generate all the seeds? . Lets say ,if the seed gets repeated after 10 iterations, then after the 10 iterations, same values are repeated. So $random is circulating inside a chain of 10 numbers.
The following example demonstrates how $random misses many seeds. I tried to display the seeds between 0 to 20 in the chain formed by initial seed of 0. Results show that total possible seeds are 4294967295 , and number of seeds possible in seed chain are 4030768279 , so we are missing some seeds. Look at the seeds between 0 to 20. Seed == 1 is missing.
EXAMPLE:
module Tb();
integer num,seed,j;
reg [0:31] i;
initial
begin
i = 0;
seed = 1;
while (seed != 0)
begin
if(i == 0)
seed = 0;
i = i + 1;
num = $random(seed);
if(seed < 20 && seed > 0)
$display(" seed is %d after values %d ",seed,i);
end
$display(" seed is one after this number of random numbers %0d total numbers available are %d",i,{32'hffff_ffff});
end
endmodule
RESULTS:
seed is 10 after values 93137101
seed is 17 after values 307298440
seed is 2 after values 410139893
seed is 12 after values 483530075
seed is 19 after values 592243262
seed is 3 after values 720224974
seed is 11 after values 1342230278
seed is 15 after values 2032553666
seed is 7 after values 2266624778
seed is 13 after values 2362534380
seed is 5 after values 2512466932
seed is 9 after values 2575033104
seed is 16 after values 2988686279
seed is 4 after values 3173376451
seed is 6 after values 3483433473
seed is 8 after values 3547878575
seed is 14 after values 3663208793
seed is 18 after values 3930700709
seed is zero after this number of random numbers 4030768279 total numbers available are 4294967295
If you want more control over randomizing real numbers in terms of sign, exponential and mantissa, use $bitstoreal() as shown in example below. For positive numbers, use sgn = 0 etc.
EXAMPLE:
module Tb();
reg sgn;
reg [10:0] exp; reg [51:0] man;
real r_num;
initial
begin
repeat(5)
begin
sgn = $random;
exp = $random;
man = $random;
r_num = $bitstoreal({sgn,exp,man});
$display("r_num = %e",r_num);
end
end
endmodule
RESULTS:
r_num = 3.649952e+193
r_num = -1.414950e-73
r_num = -3.910319e-149
r_num = -4.280878e-196
r_num = -4.327791e+273
Sometimes it is required to generate random numbers without repetition. The random numbers should be unique. For example, to generate 10 random numbers b/w 0 to 9 without repetition, the following logic can be used.
EXAMPLE:
module Tb();
integer num,i,j,index;
integer arr[9:0];
reg ind[9:0];
reg got;
initial
begin
index=0;
for(i=0;i<10;i=i+1)
begin
arr[i] = i;
ind[i] = 1;
end
for(j = 0;j<10 ;j=j+1)
begin
got = 0;
while(got == 0)
begin
index = { $random() } % 10;
if(ind[index] == 1)
begin
ind[index] = 0;
got = 1;
num = arr[index];
end
end
$write("| num=%2d |",num);
end
end
endmodule
RESULT:
| num= 8 || num= 7 || num= 5 || num= 2 || num= 1 || num= 9 || num= 6 || num= 4 || num= 0 || num= 3 |
Random number system function has a argument called seed. The seed parameter controls the numbers that $random returns such that different seeds generate different random streams. The seed parameter shall be either a reg, an integer, or a time variable. The seed value should be assigned to this variable prior to calling $random. For each system function, the seed parameter is an in-out parameter; that is, a value is passed to the function
and a different value is returned.
EXAMPLE:
module Tb();
integer num,seed,i,j;
initial
begin
for(j = 0;j<4 ;j=j+1)
begin
seed = j;
$display(" seed is %d",seed);
for(i = 0;i < 10; i=i+1)
begin
num = { $random(seed) } % 10;
$write("| num=%2d |",num);
end
$display(" ");
end
end
endmodule
RESULT:
seed is 0
| num= 8 || num= 7 || num= 7 || num= 7 || num= 7 || num= 7 || num= 5 || num= 2 || num= 1 || num= 9 |
seed is 1
| num= 8 || num= 8 || num= 2 || num= 2 || num= 6 || num= 3 || num= 8 || num= 5 || num= 5 || num= 5 |
seed is 2
| num= 8 || num= 1 || num= 0 || num= 5 || num= 0 || num= 8 || num= 6 || num= 7 || num= 1 || num= 6 |
seed is 3
| num= 8 || num= 2 || num= 2 || num= 3 || num= 8 || num= 6 || num= 1 || num= 4 || num= 3 || num= 9 |
The $random function has its own implicit variable as seed when the used is not giving explicitly giving seed. The following example shows that seed = 0 and implicit seed are having same sequence. It means that the implicitly taken seed is also 0.
EXAMPLE:
module Tb();
integer num,seed,i,j;
initial
begin
seed = 0;
for(j = 0;j<2 ;j=j+1)
begin
if(j ==0)
$display(" seed is %d",seed);
else
$display(" No seed is given ");
for(i = 0;i < 10; i=i+1)
begin
if( j == 0)
num = { $random(seed) } % 10;
else
num = { $random() } % 10;
$write("| num=%2d |",num);
end
$display(" ");
end
end
endmodule
RESULT:
seed is 0
| num= 8 || num= 7 || num= 7 || num= 7 || num= 7 || num= 7 || num= 5 || num= 2 || num= 1 || num= 9 |
No seed is given
| num= 8 || num= 7 || num= 7 || num= 7 || num= 7 || num= 7 || num= 5 || num= 2 || num= 1 || num= 9 |
The system functions shall always return the same value given the same seed. This facilitates debugging by making the operation of the system repeatable. The argument for the seed parameter should be an integer variable that is initialized by the user and only updated by the system function. This ensures the desired distribution is achieved.
EXAMPLE:
module Tb();
integer num,seed,i,j;
initial
begin
for(j = 0;j<4 ;j=j+1)
begin
seed = 2;
$display(" seed is %d",seed);
for(i = 0;i < 10; i=i+1)
begin
num = { $random(seed) } % 10;
$write("| num=%2d |",num);
end
$display(" ");
end
end
endmodule
RESULT:
seed is 2
| num= 8 || num= 1 || num= 0 || num= 5 || num= 0 || num= 8 || num= 6 || num= 7 || num= 1 || num= 6 |
seed is 2
| num= 8 || num= 1 || num= 0 || num= 5 || num= 0 || num= 8 || num= 6 || num= 7 || num= 1 || num= 6 |
seed is 2
| num= 8 || num= 1 || num= 0 || num= 5 || num= 0 || num= 8 || num= 6 || num= 7 || num= 1 || num= 6 |
seed is 2
| num= 8 || num= 1 || num= 0 || num= 5 || num= 0 || num= 8 || num= 6 || num= 7 || num= 1 || num= 6 |
Seed is inout port. Random number system function returns a random number and also returns a random number to seed inout argument also. The results of the following example demonstrates how the seed value is getting changed.
EXAMPLE:
module Tb();
integer num,seed,i,j;
initial
begin
seed = 0;
for(j = 0;j<10 ;j=j+1)
begin
num = { $random(seed) } % 10;
$write("| num=%2d |",num);
$display(" seed is %d ",seed);
end
end
endmodule
RESULT:
| num= 8 | seed is -1844104698
| num= 7 | seed is 1082744015
| num= 7 | seed is 75814084
| num= 7 | seed is 837833973
| num= 7 | seed is -2034665166
| num= 7 | seed is -958425333
| num= 5 | seed is 851608272
| num= 2 | seed is 154620049
| num= 1 | seed is -2131500770
| num= 9 | seed is -2032678137
From the above results we can make a table of seed values and return values of $random. If a seed is taken from the table, then rest of the sequence has to follow sequence in table.
Table is as falows for initial seed 0;
| num= 8 | seed is -1844104698
| num= 7 | seed is 1082744015
| num= 7 | seed is 75814084
| num= 7 | seed is 837833973
| num= 7 | seed is -2034665166
| num= 7 | seed is -958425333
| num= 5 | seed is 851608272
| num= 2 | seed is 154620049
| num= 1 | seed is -2131500770
| num= 9 | seed is -2032678137
.
.
.
.
.
table goes on........
In the following example, the seed is 837833973, which is the 4 th seed from the above table.
EXAMPLE:
module Tb();
integer num,seed,i,j;
initial
begin
seed = 837833973;
for(j = 0;j<10 ;j=j+1)
begin
num = { $random(seed) } % 10;
$write("| num=%2d |",num);
$display(" seed is %d ",seed);
end
end
endmodule
RESULTS:
| num= 7 | seed is -2034665166
| num= 7 | seed is -958425333
| num= 5 | seed is 851608272
| num= 2 | seed is 154620049
| num= 1 | seed is -2131500770
| num= 9 | seed is -2032678137
| num= 8 | seed is -1155272804
| num= 7 | seed is -1634874387
| num= 9 | seed is -153856566
| num= 2 | seed is -970066749
From the above example we can come to conclusion that $random is not giving a random number. It is randomizing seed and returning corresponding number for that seed.
Total possible seed values are 4294967295. Is it possible for $random to generate all the seeds? . Lets say ,if the seed gets repeated after 10 iterations, then after the 10 iterations, same values are repeated. So $random is circulating inside a chain of 10 numbers.
The following example demonstrates how $random misses many seeds. I tried to display the seeds between 0 to 20 in the chain formed by initial seed of 0. Results show that total possible seeds are 4294967295 , and number of seeds possible in seed chain are 4030768279 , so we are missing some seeds. Look at the seeds between 0 to 20. Seed == 1 is missing.
EXAMPLE:
module Tb();
integer num,seed,j;
reg [0:31] i;
initial
begin
i = 0;
seed = 1;
while (seed != 0)
begin
if(i == 0)
seed = 0;
i = i + 1;
num = $random(seed);
if(seed < 20 && seed > 0)
$display(" seed is %d after values %d ",seed,i);
end
$display(" seed is one after this number of random numbers %0d total numbers available are %d",i,{32'hffff_ffff});
end
endmodule
RESULTS:
seed is 10 after values 93137101
seed is 17 after values 307298440
seed is 2 after values 410139893
seed is 12 after values 483530075
seed is 19 after values 592243262
seed is 3 after values 720224974
seed is 11 after values 1342230278
seed is 15 after values 2032553666
seed is 7 after values 2266624778
seed is 13 after values 2362534380
seed is 5 after values 2512466932
seed is 9 after values 2575033104
seed is 16 after values 2988686279
seed is 4 after values 3173376451
seed is 6 after values 3483433473
seed is 8 after values 3547878575
seed is 14 after values 3663208793
seed is 18 after values 3930700709
seed is zero after this number of random numbers 4030768279 total numbers available are 4294967295
|
本文地址：http://114er.blogspot.com/2010/12/ztverilogsystem-function-random-myth-28_08.html 原创文章如转载，请注明链接： 转自Welcome Funny Guys |
0 评论: