GETNIB D,{#}S,#N Get nibble N of S into D. D = {28'b0, S.NIBBLE[N]). Syntax Question

Bob Drury

GETNIB D,{#}S,#N
Get nibble N of S into D. D = {28'b0, S.NIBBLE[N]).

Instruction works very simiar to SETNIB no issue using it. But what does " D = {28'b0, S.NIBBLE[N])."
mean? Does 28'b0 somehow mean 28 original bit s masked with the addition of S.NIBBLE[N] does some one have a description of this notation. and should last bracket be" }" not ")" or is this part of the notation?
Regards and Thanks
Bob (WRD)

evanh

That's Verilog HDL code from the chip design files. It means 28 bits of zeros concatenated with 4 bits from S, in order, from left as most significant, to right as least significant.

evanh

Yeah, the mismatched brackets do seem wrong to me too. Looking at the spreadsheet I see there many such duplications in that group of instructions. But further down there is examples of curly's being used for both opening and closing.

Update: I think that syntax muddle probably came about due to Verilog using curly's for concatenation but in the instruction sheet they indicate an option list instead as well.

Update2: Okay, more reading, parentheses seem to be only for precedence, same as general maths. So I guess it should be matching braces in that description.

Ariba

Never seen this .NIBBLE[n] syntax in Verilog. Maybe it's pseudo verilog or Chip has defined a macro for it.
I would write it in Verilog like this:

D = {28'b0, S[N*4+3:N*4]};

Andy

evanh

Yeah, I saw it as the 28b0 being the central question Bob was asking. The syntax is mixed, for sure.

Bob Drury

The D= {28'b0, S[N4+3:N4]} accounts for the position of bits to add . I am going to put this into my notes . Not knowing verilog the {,} operation is concantenation function wouldn' that
result in 00000000_00000000_00000000_0000_S[N4:+3:N4] eg N=6 should be 00000000_S[N4+3:N4]_0000_00000000_00000000 does "28'b0" with apostrophe somehow account for this when
you interpet this.
Regards and Thanks
Bob (WRD)

evanh

The S[N:M] means the parameters inside the square brackets are applied to S, not D. So, what part of S gets used for the least four bits of D is what changes.

Bob Drury

Testing GETNIB D,S,#N S =N7N6N5N4N3N2N1N0 it doesn' seem to matter which nibble is written from S it is always placed in Nibble N0 of D . D = 00000000_00000000_00000000_0000_ssss
this seems to match D = {28'b0,S[N4+3:N4]}

Testing "GETNIB D" (using "ALTGN D,{#}S" Alter subsequent GETNIB/ROLNIB instruction. Next S field = (D[11:3] + S) & $1FF, N field = D[2:0]. D += sign-extended S[17:9]) S =N7N6N5N4N3N2N1N0 it doesn' seem to matter which nibble is written from S it is always placed in Nibble N0 of D . D = 00000000_00000000_00000000_0000_ssss D = 00000000_00000000_00000000_0000_ssss
this seems to match D = {28'b0,S[N4+3:N4]}

Everything resolved
Regards and Thanks
Bob (WRD)

Bob Drury

woops corected Testing "GETNIB D" (using "ALTGN D,{#}S) result the same for syntax
Bob (WRD)