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// ============================================================================
//        __
//   \\__/ o\    (C) 2015-2022  Robert Finch, Waterloo
//    \  __ /    All rights reserved.
//     \/_//     robfinch<remove>@finitron.ca
//       ||
//
//
// BSD 3-Clause License
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// ============================================================================
//
// Compute the graphics address
//
import gfx_pkg::*;

module gfx_calc_address(clk, base_address_i, color_depth_i, bmp_width_i, x_coord_i, y_coord_i,
        address_o, mb_o, me_o, ce_o);
parameter SW = 96;              // strip width in bits
parameter BN = 6;
input clk;
input [31:0] base_address_i;
input [3:0] color_depth_i;
input [15:0] bmp_width_i;       // pixel per line
input [15:0] x_coord_i;
input [15:0] y_coord_i;
output [31:0] address_o;
output [BN:0] mb_o;                                     // mask begin
output [BN:0] me_o;                                     // mask end
output [BN:0] ce_o;                                     // color bits end

// This coefficient is a fixed point fraction representing the inverse of the
// number of pixels per strip. The inverse (reciprocal) is used for a high
// speed divide operation.
reg [15:0] coeff;
always @(color_depth_i)
        case(color_depth_i)
        BPP6: coeff = 65536*6/SW;
        BPP8:   coeff = 65536*8/SW;
        BPP12:  coeff = 65536*12/SW;
        BPP16:  coeff = 65536*16/SW;
        BPP18:  coeff = 65536*18/SW;
        BPP21:  coeff = 65536*21/SW;
        BPP24:  coeff = 65536*24/SW;
        BPP27:  coeff = 65536*27/SW;
        BPP32:  coeff = 65536*32/SW;
        BPP33:  coeff = 65536*33/SW;
        default:        coeff = 65536*16/SW;
        endcase

// Bits per pixel minus one.
reg [5:0] bpp;
always @(color_depth_i)
case(color_depth_i)
BPP6: bpp = 5;
BPP8:   bpp = 7;
BPP12: bpp = 11;
BPP16:  bpp = 15;
BPP18:  bpp = 17;
BPP21:  bpp = 20;
BPP24:  bpp = 23;
BPP27:  bpp = 26;
BPP32:  bpp = 31;
BPP33:  bpp = 32;
default:        bpp = 15;
endcase

// Color bits per pixel minus one.
reg [5:0] cbpp;
always @(color_depth_i)
case(color_depth_i)
BPP6: cbpp = 2;
BPP8:   cbpp = 4;
BPP12: cbpp = 8;
BPP16:  cbpp = 11;
BPP18:  cbpp = 14;
BPP21:  cbpp = 17;
BPP24:  cbpp = 20;
BPP27:  cbpp = 23;
BPP32:  cbpp = 26;
BPP33:  cbpp = 29;
default:        cbpp = 11;
endcase

// This coefficient is the number of bits used by all pixels in the strip. 
// Used to determine pixel placement in the strip.
reg [7:0] coeff2;
always @(color_depth_i)
case(color_depth_i)
BPP6: coeff2 = SW-(SW % 6);
BPP8:   coeff2 = SW-(SW % 8);
BPP12: coeff2 = SW-(SW % 12);
BPP16:  coeff2 = SW-(SW % 16);
BPP18:  coeff2 = SW-(SW % 18);
BPP21:  coeff2 = SW-(SW % 21);
BPP24:  coeff2 = SW-(SW % 24);
BPP27:  coeff2 = SW-(SW % 27);
BPP32:  coeff2 = SW-(SW % 32);
BPP33:  coeff2 = SW-(SW % 33);
default:        coeff2 = SW-(SW % 16);
endcase

// Compute the fixed point horizonal strip number value. This has 16 binary
// point places.
wire [31:0] strip_num65k = x_coord_i * coeff;
// Truncate off the binary fraction to get the strip number. The strip
// number will be used to form part of the address.
wire [17:0] strip_num = strip_num65k[31:16];
// Calculate pixel position within strip using the fractional part of the
// horizontal strip number.
wire [15:0] strip_fract = strip_num65k[15:0]+16'h7F;  // +7F to round
// Pixel beginning bit is ratio of pixel # into all bits used by pixels
wire [15:0] ndx = strip_fract[15:7] * coeff2;
assign mb_o = ndx[15:9];  // Get whole pixel position (discard fraction)
assign me_o = mb_o + bpp; // Set high order position for mask
assign ce_o = mb_o + cbpp;
// num_strips is essentially a constant value unless the screen resolution changes.
// Gain performance here by regstering the multiply so that there aren't two
// cascaded multiplies when calculating the offset.
reg [31:0] num_strips65k;
always @(posedge clk)
        num_strips65k <= bmp_width_i * coeff;
wire [15:0] num_strips = num_strips65k[31:16];

wire [31:0] offset = {(({4'b0,num_strips} * y_coord_i) + strip_num),4'h0};

assign address_o = base_address_i + offset;

endmodule