弱问一个简单的问题

You ask a very good question.
According IEEE 1364-2005 page 341:
The system function $random provides a mechanism for generating random numbers. The function returns
a new 32-bit random number each time it is called. The random number is a signed integer; it can be positive
or negative. For further information on probabilistic random number generators, see 17.9.2.
The seed argument controls the numbers that $random returns so that different seeds generate different
random streams. The seed argument shall be either a reg, an integer, or a time variable. The seed value
should be assigned to this variable prior to calling $random.
For example:
Example 1—Where b is greater than 0, the expression ($random % b) gives a number in the following
range: [(-b+1): (b-1)].
So I think your assumption is wrong.

我是看到夏宇闻老师书上他是这么写的，因为我不是很明白，所以才问的。

很简单，说明书错了：）

好好干，你能成为一个优秀的工程师的.

夏宇闻 is right. I make a mistake.
The {} treats all parameter as unsigned  value and so products an  unsigned value too.
You can run the following test program in simulator.
You can try to remove {} to watch the different effect.

// test program
module top;      
   initial       repeat(80) #1 $display("%d ($random) mod 60  ->  %d", $random, {$random}%60);   
   initial       #100 $finish;           
endmodule

上海保洁公司

回复
对这个系统函数，我还没有用过。不过现在学到了一些新的知识了。

恩，不错的问题

汗。这个人家就是说加了{}只有正，没加正负都有，你知道怎么用就行了撒，你还去研究为什么啊?这个是人家定的标准撒，就像文件是用"..."一样。

楼上的，话是这么说，不过多思考一番就会发现，标准的定义是连贯同意的，对于初学者还是很有好处的，支持提问的！