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array access thorugh a variable (non costant expression)

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I have this metodology issue.
The access to an array (index of the array) is allowed only to be  a constant expression.
In case of a parametric array (the size of the array is a parameter) in order to mux a sub-part of the vector I use an expression of this type:

i_rdata[(px_max-cmd_exe_cnt_d1*shm_width)*px_width-1:(px_max-cmd_exe_cnt_d1*shm_width-shm_width)*px_width] = rdata_shram;

where cmd_exe_cnt_d1 is a variable and others are constant.

Obviously I get an error about the usage of a non costant expression, following reported.


Compiling SHM component...
file: /IPREUSE/DATABASE/INTERNAL/DIG/INPROGRESS/gborgo/simd_A0_a/config/../generic/rtl/svlog/shm.v
    i_rdata[(px_max-cmd_exe_cnt_d1*shm_width)*px_width-1:(px_max-cmd_exe_cnt_d1*shm_width-shm_width)*px_width] = rdata_shram;
                                 |
ncvlog: *E,NOTPAR (/IPREUSE/DATABASE/INTERNAL/DIG/INPROGRESS/gborgo/simd_A0_a/config/../generic/rtl/svlog/shm.v,377|33): Illegal operand for constant expression [4(IEEE)].
    i_rdata[(px_max-cmd_exe_cnt_d1*shm_width)*px_width-1:(px_max-cmd_exe_cnt_d1*shm_width-shm_width)*px_width] = rdata_shram;

I found a simple work-around using a case structure:

      unique case (cmd_exe_cnt_d1)
        0      : i_rdata[(px_max-0*shm_width)*px_width-1:(px_max-0*shm_width-shm_width)*px_width] = rdata_shram;
        1      : i_rdata[(px_max-1*shm_width)*px_width-1:(px_max-1*shm_width-shm_width)*px_width] = rdata_shram;

....
...
...

        31     : i_rdata[(px_max-1*shm_width)*px_width-31:(px_max-31*shm_width-shm_width)*px_width] = rdata_shram;
        default:  i_rdata = 0;
      endcase   


but in this case I have an implicit limitation on the upper limit of the vector size. In this case 31 elements.

Is there any other mode to address this methodology issue ?

The code must be synthesizable.

Thanks
BR
Giampiero


Originally posted in cdnusers.org by borgonov69

  • When you use the [a:b] syntax to select a slice of a vector, both expressions need to be compile-time constants. But you can vary the bit range if you specify the left-hand-bit using a variable and keep a fixed width using [a -: width] or [a +: width].

    i_rdata[(px_max-cmd_exe_cnt_d1*shm_width)*px_width-1-:px_width] = rdata_shram;


    Originally posted in cdnusers.org by TAM

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