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## [/] [zipcpu/] [trunk/] [rtl/] [core/] [div.v] - Rev 205

```////////////////////////////////////////////////////////////////////////////////
//
// Filename:	div.v
//
// Project:	Zip CPU -- a small, lightweight, RISC CPU soft core
//
// Purpose:	Provide an Integer divide capability to the Zip CPU.  Provides
//		for both signed and unsigned divide.
//
// Steps:
//	i_rst	The DIVide unit starts in idle.  It can also be placed into an
//	idle by asserting the reset input.
//
//	i_wr	When i_rst is asserted, a divide begins.  On the next clock:
//
//	  o_busy is set high so everyone else knows we are at work and they can
//		wait for us to complete.
//
//	  pre_sign is set to true if we need to do a signed divide.  In this
//		case, we take a clock cycle to turn the divide into an unsigned
//		divide.
//
//	  o_quotient, a place to store our result, is initialized to all zeros.
//
//	  r_dividend is set to the numerator
//
//	  r_divisor is set to 2^31 * the denominator (shift left by 31, or add
//		31 zeros to the right of the number.
//
//	pre_sign When true (clock cycle after i_wr), a clock cycle is used
//		to take the absolute value of the various arguments (r_dividend
//		and r_divisor), and to calculate what sign the output result
//		should be.
//
//
//	At this point, the divide is has started.  The divide works by walking
//	through every shift of the
//
//		    DIVIDEND	over the
//		DIVISOR
//
//	If the DIVISOR is bigger than the dividend, the divisor is shifted
//	right, and nothing is done to the output quotient.
//
//		    DIVIDEND
//		 DIVISOR
//
//	This repeats, until DIVISOR is less than or equal to the divident, as in
//
//		DIVIDEND
//		DIVISOR
//
//	At this point, if the DIVISOR is less than the dividend, the
//	divisor is subtracted from the dividend, and the DIVISOR is again
//	shifted to the right.  Further, a '1' bit gets set in the output
//	quotient.
//
//	Once we've done this for 32 clocks, we've accumulated our answer into
//	the output quotient, and we can proceed to the next step.  If the
//	result will be signed, the next step negates the quotient, otherwise
//	it returns the result.
//
//	On the clock when we are done, o_busy is set to false, and o_valid set
//	to true.  (It is a violation of the ZipCPU internal protocol for both
//	busy and valid to ever be true on the same clock.  It is also a
//	violation for busy to be false with valid true thereafter.)
//
//
// Creator:	Dan Gisselquist, Ph.D.
//		Gisselquist Technology, LLC
//
////////////////////////////////////////////////////////////////////////////////
//
// Copyright (C) 2015-2017, Gisselquist Technology, LLC
//
// This program is free software (firmware): you can redistribute it and/or
// modify it under the terms of  the GNU General Public License as published
// by the Free Software Foundation, either version 3 of the License, or (at
// your option) any later version.
//
// This program is distributed in the hope that it will be useful, but WITHOUT
// ANY WARRANTY; without even the implied warranty of MERCHANTIBILITY or
// FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
// for more details.
//
// You should have received a copy of the GNU General Public License along
// with this program.  (It's in the \$(ROOT)/doc directory.  Run make with no
// target there if the PDF file isn't present.)  If not, see
//
// License:	GPL, v3, as defined and found on www.gnu.org,
//
//
////////////////////////////////////////////////////////////////////////////////
//
//
// `include "cpudefs.v"
//
module	div(i_clk, i_rst, i_wr, i_signed, i_numerator, i_denominator,
o_busy, o_valid, o_err, o_quotient, o_flags);
parameter		BW=32, LGBW = 5;
input			i_clk, i_rst;
// Input parameters
input			i_wr, i_signed;
input	[(BW-1):0]	i_numerator, i_denominator;
// Output parameters
output	reg		o_busy, o_valid, o_err;
output	reg [(BW-1):0]	o_quotient;
output	wire	[3:0]	o_flags;

// r_busy is an internal busy register.  It will clear one clock
// before we are valid, so it can't be o_busy ...
//
reg			r_busy;
reg	[(2*BW-2):0]	r_divisor;
reg	[(BW-1):0]	r_dividend;
wire	[(BW):0]	diff; // , xdiff[(BW-1):0];
assign	diff = r_dividend - r_divisor[(BW-1):0];
// assign	xdiff= r_dividend - { 1'b0, r_divisor[(BW-1):1] };

reg		r_sign, pre_sign, r_z, r_c, last_bit;
reg	[(LGBW-1):0]	r_bit;
reg	zero_divisor;

// The Divide logic begins with r_busy.  We use r_busy to determine
// whether or not the divide is in progress, vs being complete.
// Here, we clear r_busy on any reset and set it on i_wr (the request
// do to a divide).  The divide ends when we are on the last bit,
// or equivalently when we discover we are dividing by zero.
initial	r_busy = 1'b0;
always @(posedge i_clk)
if (i_rst)
r_busy <= 1'b0;
else if (i_wr)
r_busy <= 1'b1;
else if ((last_bit)||(zero_divisor))
r_busy <= 1'b0;

// o_busy is very similar to r_busy, save for some key differences.
// Primary among them is that o_busy needs to (possibly) be true
// for an extra clock after r_busy clears.  This would be that extra
// clock where we negate the result (assuming a signed divide, and that
// the result is supposed to be negative.)  Otherwise, the two are
// identical.
initial	o_busy = 1'b0;
always @(posedge i_clk)
if (i_rst)
o_busy <= 1'b0;
else if (i_wr)
o_busy <= 1'b1;
else if (((last_bit)&&(~r_sign))||(zero_divisor))
o_busy <= 1'b0;
else if (~r_busy)
o_busy <= 1'b0;

// If we are asked to divide by zero, we need to halt.  The sooner
// we halt and report the error, the better.  Hence, here we look
// for a zero divisor while being busy.  The always above us will then
// look at this and halt a divide in the middle if we are trying to
// divide by zero.
//
// Note that this works off of the 2BW-1 length vector.  If we can
// simplify that, it should simplify our logic as well.
initial	zero_divisor = 1'b0;
always @(posedge i_clk)
// zero_divisor <= (r_divisor == 0)&&(r_busy);
if (i_rst)
zero_divisor <= 1'b0;
else if (i_wr)
zero_divisor <= (i_denominator == 0);
else if (!r_busy)
zero_divisor <= 1'b0;

// o_valid is part of the ZipCPU protocol.  It will be set to true
// anytime our answer is valid and may be used by the calling module.
// Indeed, the ZipCPU will halt (and ignore us) once the i_wr has been
// set until o_valid gets set.
//
// Here, we clear o_valid on a reset, and any time we are on the last
// bit while busy (provided the sign is zero, or we are dividing by
// zero).  Since o_valid is self-clearing, we don't need to clear
// it on an i_wr signal.
initial	o_valid = 1'b0;
always @(posedge i_clk)
if (i_rst)
o_valid <= 1'b0;
else if (r_busy)
begin
if ((last_bit)||(zero_divisor))
o_valid <= (zero_divisor)||(!r_sign);
end else if (r_sign)
begin
o_valid <= (!zero_divisor); // 1'b1;
end else
o_valid <= 1'b0;

// Division by zero error reporting.  Anytime we detect a zero divisor,
// we set our output error, and then hold it until we are valid and
// everything clears.
initial	o_err = 1'b0;
always @(posedge i_clk)
if((i_rst)||(o_valid))
o_err <= 1'b0;
else if (((r_busy)||(r_sign))&&(zero_divisor))
o_err <= 1'b1;
else
o_err <= 1'b0;

// r_bit
//
// Keep track of which "bit" of our divide we are on.  This number
// ranges from 31 down to zero.  On any write, we set ourselves to
// 5'h1f.  Otherwise, while we are busy (but not within the pre-sign
// adjustment stage), we subtract one from our value on every clock.
always @(posedge i_clk)
if ((r_busy)&&(!pre_sign))
r_bit <= r_bit + {(LGBW){1'b1}};
else
r_bit <= {(LGBW){1'b1}};

// last_bit
//
// This logic replaces a lot of logic that was inside our giant state
// machine with ... something simpler.  In particular, we'll use this
// logic to determine we are processing our last bit.  The only trick
// is, this bit needs to be set whenever (r_busy) and (r_bit == 0),
// hence we need to set on (r_busy) and (r_bit == 1) so as to be set
// when (r_bit == 0).
initial	last_bit = 1'b0;
always @(posedge i_clk)
if (r_busy)
last_bit <= (r_bit == {{(LGBW-1){1'b0}},1'b1});
else
last_bit <= 1'b0;

// pre_sign
//
// This is part of the state machine.  pre_sign indicates that we need
// a extra clock to take the absolute value of our inputs.  It need only
// be true for the one clock, and then it must clear itself.
initial	pre_sign = 1'b0;
always @(posedge i_clk)
if (i_wr)
pre_sign <= i_signed;
else
pre_sign <= 1'b0;

// As a result of our operation, we need to set the flags.  The most
// difficult of these is the "Z" flag indicating that the result is
// zero.  Here, we'll use the same logic that sets the low-order
// bit to clear our zero flag, and leave the zero flag set in all
// other cases.  Well ... not quite.  If we need to flip the sign of
// our value, then we can't quite clear the zero flag ... yet.
always @(posedge i_clk)
if((r_busy)&&(r_divisor[(2*BW-2):(BW)] == 0)&&(!diff[BW]))
// If we are busy, the upper bits of our divisor are
// zero (i.e., we got the shift right), and the top
// (carry) bit of the difference is zero (no overflow),
// then we could subtract our divisor from our dividend
// and hence we add a '1' to the quotient, while setting
// the zero flag to false.
r_z <= 1'b0;
else if ((!r_busy)&&(!r_sign))
r_z <= 1'b1;

// r_dividend
// This is initially the numerator.  On a signed divide, it then becomes
// the absolute value of the numerator.  We'll subtract from this value
// the divisor shifted as appropriate for every output bit we are
// looking for--just as with traditional long division.
always @(posedge i_clk)
if (pre_sign)
begin
// If we are doing a signed divide, then take the
// absolute value of the dividend
if (r_dividend[BW-1])
r_dividend <= -r_dividend;
// The begin/end block is important so we don't lose
// the fact that on an else we don't do anything.
end else if((r_busy)&&(r_divisor[(2*BW-2):(BW)]==0)&&(!diff[BW]))
// This is the condition whereby we set a '1' in our
// output quotient, and we subtract the (current)
// divisor from our dividend.  (The difference is
// already kept in the diff vector above.)
r_dividend <= diff[(BW-1):0];
else if (!r_busy)
// Once we are done, and r_busy is no longer high, we'll
// always accept new values into our dividend.  This
// guarantees that, when i_wr is set, the new value
// is already set as desired.
r_dividend <=  i_numerator;

initial	r_divisor = 0;
always @(posedge i_clk)
if (pre_sign)
begin
if (r_divisor[(2*BW-2)])
r_divisor[(2*BW-2):(BW-1)]
<= -r_divisor[(2*BW-2):(BW-1)];
end else if (r_busy)
r_divisor <= { 1'b0, r_divisor[(2*BW-2):1] };
else
r_divisor <= {  i_denominator, {(BW-1){1'b0}} };

// r_sign
// is a flag for our state machine control(s).  r_sign will be set to
// true any time we are doing a signed divide and the result must be
// negative.  In that case, we take a final logic stage at the end of
// the divide to negate the output.  This flag is what tells us we need
// to do that.  r_busy will be true during the divide, then when r_busy
// goes low, r_sign will be checked, then the idle/reset stage will have
// been reached.  For this reason, we cannot set r_sign unless we are
// up to something.
initial	r_sign = 1'b0;
always @(posedge i_clk)
if (pre_sign)
r_sign <= ((r_divisor[(2*BW-2)])^(r_dividend[(BW-1)]));
else if (r_busy)
r_sign <= (r_sign)&&(!zero_divisor);
else
r_sign <= 1'b0;

always @(posedge i_clk)
if (r_busy)
begin
o_quotient <= { o_quotient[(BW-2):0], 1'b0 };
if ((r_divisor[(2*BW-2):(BW)] == 0)&&(!diff[BW]))
begin
o_quotient <= 1'b1;
end
end else if (r_sign)
o_quotient <= -o_quotient;
else
o_quotient <= 0;

// Set Carry on an exact divide
// Perhaps nothing uses this, but ... well, I suppose we could remove
// this logic eventually, just ... not yet.
always @(posedge i_clk)
r_c <= (r_busy)&&((diff == 0)||(r_dividend == 0));

// The last flag: Negative.  This flag is set assuming that the result
// of the divide was negative (i.e., the high order bit is set).  This
// will also be true of an unsigned divide--if the high order bit is
// ever set upon completion.  Indeed, you might argue that there's no
// logic involved.
wire	w_n;
assign w_n = o_quotient[(BW-1)];

assign o_flags = { 1'b0, w_n, r_c, r_z };
endmodule
```