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`timescale 1ns / 1ps

//////////////////////////////////////////////////////////////////////////////////

// Company:

// Engineer:

//

// Create Date:    12:02:03 10/22/2006

// Design Name:

// Module Name:    vgachr

// Project Name:

// Target Devices:

// Tool versions:

// Description:

//

// Dependencies:

//

// Revision:

// Revision 0.01 - File Created

// Additional Comments:

//

// Simulation plugs exist in this code. Look for "????? SIMULATION PLUG"

//

// Debug plugs exist in this code. Look for "????? DEBUG PLUG"

//

//////////////////////////////////////////////////////////////////////////////////

////////////////////////////////////////////////////////////////////////////////

//

// TERMINAL EMULATOR

//

// Emulates an ADM 3A dumb terminal, with a MITS serial I/O board interface.

// Two ports are emulated:

//

// 0: Control

// 1: Data

//

// The MITS serial card has all of its configuration performed by jumpers on the

// card, which means there is no programming configuration required. The data

// to the terminal is sent out of the data port, while the data from the

// keyboard, which is not yet implemented, is read from the same data port.

// The parity is ignored on output, and set to 0 on input.

//

// The control register ignores all writes, and returns $80 if the terminal is

// busy, otherwise $00. This is the output ready to send bit. The busy bit

// reflects if the state machine is processing an operation. This is a cheat

// that only works with our emulated terminal/serial board pair, because

// normally there is no way for the local CPU to know that the remote terminal

// is busy. This can lead to problems in the real world, and it's why Unix

// "termcap" terminal descriptions commonly have waiting periods perscribed

// time consuming operations like screen clear. This emulated terminal is

// "ideal" in that it accounts for all of this automatically, but it does not

// hurt the realisim of the emulation. An application that performs delays based

// on real terminal operations won't be incorrect because it performs a delay,

// but a stupid application that relies on this "smart" implementation might

// fail to run on the real thing.

//

// The ADM 3A terminal emulation is based on "ADM-3A Operators manual" of 1979,

// and "ADM 3A Dumb Terminal Users Reference Manual" of April, 1986, and the

// Unix termcap definition of the terminal.

//

// Several actions of terminals are typically not listed in the documentation,

// and the ADM 3A is no different. That's why termcap exists. The following

// actions were derived from the termcap definition:

//

// 1. The screen clear command also homes the cursor.

// 2. Giving the terminal a right cursor command while at the right side of the

// 3. Screen wraps the cursor around to the right, one line down. If the cursor

//    is at the 80th collumn of the 24th line, it will then scroll.

// 4. Down cursor (line feed) on the 24th line scrolls the screen.

//

// Note that rule (3) causes the terminal to be unable to ever write a character

// to the collumn 80 character of line 24, which is interesting if you are

// writing a full screen editor. The ADM 3A had a switch for this behavior, but

// the user could hardly be expected to open the side panel and flip this switch

// to be able to edit.

//

// The following actions are unknown:

//

// 1. A cursor left command given when the cursor is at the left side of the

//    screen has unknown effect. According to termcap, it is not to go to the

//    end of the next line up. I have guessed here that it is to simply

//    refuse to move.

// 2. A cursor up command given when the cursor is at the top of the screen

//    has unknown effect. I have guessed here that is is to simply refuse to

//    move.

//

// These are terminal features there are no plans to implement:

//

// 1. ENQ or answerback mode.

// 2. Bell (where would it go?).

// 3. Ctrl-N and Ctrl-O keyboard locking and extention port.

// 4. Graphics modes. This was added in later ADM 3A terminals, so were unlikely

//    to be used in early software, and would take up another table.

// 5. Any of the setup modes or features, as operated by the keyboard.

//

// The ADM 3A terminal will pretty much serve as an upward compatible version

// of the ASR-33 teletypes (Western Union Surplus) that were also commonly used

// in the time of the Altair. The ADM 3A was a common replacement for such

// units.

//

// Bugs/problems/issues:

//

// 1. The emulation was occasionally observed to hang. The CPU was still running

// and polling the keyboard, but no reply is received when keys are hit. The rdy

// signal is not coming from the low level keyboard logic.

// 2. Cursor left at left hand side does not refuse to move (see above).

// 3. moving to the 80th collum on line 24 and typing a character does not cause

// the screen to scroll.

// 4. On sign-on, the first two characters are missing the top row. I suspect

// this is due to the start of frame character preloading that is done.

// 5. \ESCGc or set attributes does not work from the keyboard unless you hit

// caps lock or hold the shift key down before and after the escape. It should

// work anytime the upper case G is hit.

// 6. Need to add reduce modes.

//

//

// Terminal height and width

//

`define scnchrs 80 // width

`define scnlins 24 // height

//

// Terminal states

//

`define term_idle    5'h00 // idle

`define term_wrtstd2 5'h01 // write standard character #2

`define term_wrtstd3 5'h02 // write standard character #3

`define term_wrtstd4 5'h03 // write standard character #4

`define term_clear   5'h04 // clear screen and home cursor

`define term_clear2  5'h05 // clear screen and home cursor #2

`define term_clear3  5'h06 // clear screen and home cursor #3

`define term_clear4  5'h07 // clear screen and home cursor #4

`define term_fndstr  5'h08 // find start of current line

`define term_scroll  5'h09 // scroll screen

`define term_scroll1 5'h0a // scroll screen #1

`define term_scroll2 5'h0b // scroll screen #2

`define term_scroll3 5'h0c // scroll screen #3

`define term_scroll4 5'h0d // scroll screen #4

`define term_scroll5 5'h0e // scroll screen #5

`define term_scroll6 5'h0f // scroll screen #6

`define term_esc     5'h10 // escape

`define term_poscur  5'h11 // position cursor

`define term_poscur2 5'h12 // position cursor #2

`define term_attset  5'h13 // set screen attributes

//

// Terminal attribute bits

//

`define attr_blank   5'b00001 // blank

`define attr_blink   5'b00010 // blink

`define attr_reverse 5'b00100 // reverse

`define attr_under   5'b01000 // underline

`define attr_rinten  5'b10000 // reduced intensity

module terminal(addr, data, write, read, select, r, g, b, hsync_n, vsync_n,

                ps2_clk, ps2_data, reset, clock, diag);

   input        addr;     // control reg/data reg address

   inout [7:0]  data;     // CPU data

   input        write;    // CPU write

   input        read;     // CPU read

   input        select;   // controller select

   input        reset;    // CPU reset

   input        clock;    // CPU clock

   output [2:0] r, g, b;  // R,G,B color output buses

   output       hsync_n;  // horizontal sync pulse

   output       vsync_n;  // vertical sync pulse

   input        ps2_clk;  // clock from keyboard

   input        ps2_data; // data from keyboard

   output [7:0] diag;     // diagnostic 8 bit port

   // internal definitions

   reg [4:0]   state;   // terminal state machine

   reg [10:0]  cursor;  // cursor address

   reg [10:0]  tcursor; // cursor temp address

   reg [7:0]   chrdatw; // character write data

   reg         outrdy;  // output ready to send

   reg         wrtchr;  // character ready to write

   reg  [7:0]  datao;   // intermediate for data output

   reg  [7:0]  rowchr;  // row holding character

   reg  [4:0]  curatr;  // current attribute set

   // character map communication bus

   reg  [10:0] cmaddr;  // character map address to read or write

   reg         cmread;  // character map read from address

   reg         cmwrite; // character map write from address

   wire  [7:0] cmdata;  // character map data to be written/read

   wire  [4:0] cmattr;  // character map attributes to be written/read

   reg   [7:0] cmdatai; // character map data to be written

   reg         cmdatae; // character map data enable

   reg   [7:0] cmattri; // character map attribute to be written

   reg         cmattre; // character map attribute enable

   // keyboard communication

   wire   [7:0] scancode;   // key scancode

   wire         parity;     // keyboard parity

   wire         busy;       // busy scanning code

   wire         rdy;        // ready with code

   wire         error;      // scancode error

   reg          scnrdy;     // scancode

   reg          extkey;     // is extended key

   reg          capslock;   // caps lock toggle

   reg          leftshift;  // state of left shift key

   reg          rightshift; // state of right shift key

   reg          leftctrl;   // state of left control key

   reg          rightctrl;  // state of right control key

   reg          relcod;     // release code active

   reg          extcod;     // extention code active

   wire   [7:0] asciidata;  // output of lookup rom

   wire   [7:0] asciidatau; // output of lookup rom

   reg          clrrdy;     // clear input ready

   // here we put signals out the diagnostic port if required.

   assign diag[0] = busy;

   assign diag[1] = rdy;

   assign diag[2] = scnrdy;

   assign diag[3] = ps2_clk;

   assign diag[4] = ps2_data;

   assign diag[5] = relcod;

   assign diag[6] = leftctrl;

   assign diag[7] = rightctrl;

   // instantiate memory mapped character display

   chrmemmap display(!reset, clock, r, g, b, hsync_n, vsync_n, cmaddr, cmread,

                     cmwrite, cmdata, cmattr, cursor);

   // instantiate ps/2 keyboard. Note that the keyboard decoder only generates

   // codes on release, which has to be changed, since we need both asserts and

   // deasserts.

   ps2_kbd vgai(.clk(clock), .rst(reset), .ps2_clk(ps2_clk), .ps2_data(ps2_data),

                .scancode(scancode), .parity(parity), .busy(busy), .rdy(rdy),

                .error(error));

   // instantiate keyboard scan lookup roms

   scnrom kbdtbl(scancode, asciidata); // lower case

   scnromu kbdtblu(scancode, asciidatau); // lower case

   // process keyboard input state

   always @(posedge clock)

      if (reset) begin // perform reset actions

         leftshift <= 0; // clear left shift key state

         rightshift <= 0; // clear right shift key state

         leftctrl <= 0; // clear left control key state

         rightctrl <= 0; // clear right control key state

         capslock <= 0; // clear caps lock state

         relcod <= 0; // clear release status

         scnrdy <= 0; // clear key ready

         extkey <= 0; // is an extended key

   end else begin

      if (rdy) begin

          // if the release code $f0 occurs, set release until the next key occurs

          if (scancode == 8'hf0) relcod <= 1;

          // if the extention code $e0 occurs, set the extention flag

          else if (scancode == 8'he0) extcod <= 1;

          else if (relcod) begin // release

             relcod <= 0; // reset release code

             // reset any extention code

             if (scancode != 8'hf0 && scancode != 8'he0) extcod <= 0;

             // if caps lock is hit, toggle caps lock status

             if (scancode == 8'h58) capslock <= !capslock;

             // process left and right shift key up

             if (scancode == 8'h12) leftshift <= 0; // left up

             if (scancode == 8'h59) rightshift <= 0; // right up

             // process control key up

             if (scancode == 8'h14) begin

                if (extcod) rightctrl <= 0; // right up

                else leftctrl <= 0; // left up

             end

          end else begin // assert

             scnrdy <= 1; // set key is ready

             extkey <= extcod; // set extended status of key

             // reset any extention code

             if (scancode != 8'hf0 && scancode != 8'he0) extcod <= 0;

             // process left and right shift key down

             if (scancode == 8'h12) leftshift <= 1; // left down

             if (scancode == 8'h59) rightshift <= 1; // right down

             // process control key down

             if (scancode == 8'h14) begin

                if (extcod) rightctrl <= 1; // right down

                else leftctrl <= 1; // left down

             end

          end

      end else if (clrrdy) scnrdy <= 0; // clear key ready

   end

   // process terminal emulation

   always @(posedge clock)

      if (reset) begin // reset

         // on reset, we set the state machine to perform a screen clear and

         // home cursor

         // ????? SIMULATION PLUG

         // Don't clear screen, this takes too long in simulation

         state <= `term_clear; // set to clear screen

         // state <= `term_idle; // continue

         cursor <= 0; // set cursor to home

         outrdy <= 0; // set not ready to send

         wrtchr <= 0; // set no character to write

         cmread <= 0; // clear read character

         cmwrite <= 0; // clear write character

         cmdatae <= 0; // no enable character map data

         cmattre <= 0; // no enable character map attributes

         clrrdy <= 0; // set clear ready off

         curatr <= 5'b0; // set no attributes active

   end else begin

      if (write&select) begin // CPU write

         if (addr) begin

            chrdatw <= data & 8'h7f; // set character write data without parity

            wrtchr <= 1; // character ready to write

            outrdy <= 0; // remove ready to send

         end

      end else if (read&select) begin // CPU read

         if (addr) begin

            // Return decoded keyboard, with shifting. The letter keys, 'a'-'z',

            // can be shifted by the shift keys, but not by caps lock state.

            // The other keys can be shifted by either. This matches PC

            // behavior, which lets you have cap locks on without shifting the

            // letter keys. The control codes are shifted as a block from `($60)

            // to ~($7e) down to $00. This leaves out \US and \DEL, but these

            // codes get generated by the DEL key.

            if (extkey) begin // its an extended key

               if (scancode == 8'h75) datao <= 8'h0b; // up

               else if (scancode == 8'h6b) datao <= 8'h08; // left

               else if (scancode == 8'h72) datao <= 8'h0a; // down

               else if (scancode == 8'h74) datao <= 8'h0c; // right

               else if (scancode == 8'h6c) datao <= 8'h1e; // home

               else if (scancode == 8'h71) datao <= 8'h1f; // delete (rub out)

            end else if (asciidata >= 8'h60 && asciidata <= 8'h7e &&

                         (leftctrl || rightctrl))

               datao <= asciidata & 8'h1f;

            else if (asciidata >= 8'h61 && asciidata <= 8'h7a)

               datao <= leftshift||rightshift||capslock ? asciidatau: asciidata;

            else

               datao <= leftshift||rightshift ? asciidatau: asciidata;

            clrrdy <= 1; // clear scancode ready f/f

         end else

            datao <= (!outrdy << 7) | (scnrdy << 5); // return ready statuses

      end else clrrdy <= 0; // clear keyboard ready signal

      case (state) // run output state

         `term_idle: begin // idle waiting for character

            // We wait for the cpu cycle to end before running the state machine

            // write procedure. This allows this module to run at full speed,

            // while the rest of the CPU logic runs slow. The vga logic must run

            // at a fixed speed because it has the display to run, but the rest

            // can be slow to allow debugging.

            if (wrtchr&!(select&read|select&write)) begin

               // process character after CPU goes away

               if (chrdatw >= 8'h20 && chrdatw != 8'h7f) begin

                  // write standard (non-control) character

                  cmaddr <= cursor; // set address at cursor

                  cmdatai <= chrdatw; // place character data to write

                  cmattri <= curatr; // set with current attributes

                  cmdatae <= 1; // enable data to memory

                  cmattre <= 1; // enable attributes to memory

                  state <= `term_wrtstd2; // continue

               end else begin // control character

                  if (chrdatw == 8'h0a) begin // line down (line feed)

                     if (cursor < 23*80) begin // not at screen end

                        cursor <= cursor+80;

                        wrtchr <= 0; // remove ready to write

                        outrdy <= 1; // set ready to send

                     end else begin

                        tcursor <= 0; // set temp cursor to screen start

                        state <= `term_scroll; // go to scroll screen

                      end

                  end else if (chrdatw == 8'h0b) begin // line up

                     if (cursor >= 80) cursor <= cursor-80;

                     wrtchr <= 0; // remove ready to write

                     outrdy <= 1; // set ready to send

                  end else if (chrdatw == 8'h0d) begin // carriage return

                     tcursor <= 0; // set start of line cursor

                     state <= `term_fndstr; // go to find start of line

                  end else if (chrdatw == 8'h08) begin // character left

                     // if not at home position, back up

                     if (cursor > 0) cursor <= cursor-1;

                     wrtchr <= 0; // remove ready to write

                     outrdy <= 1; // set ready to send

                  end else if (chrdatw == 8'h0c) begin // character right

                     // if not at screen end, go forward

                     if (cursor < 80*24-1) cursor <= cursor+1;

                     wrtchr <= 0; // remove ready to write

                     outrdy <= 1; // set ready to send

                  end else if (chrdatw == 8'h1e) begin // home

                     cursor <= 0; // set to home position

                     wrtchr <= 0; // remove ready to write

                     outrdy <= 1; // set ready to send

                  end else if (chrdatw == 8'h1a) begin // clear screen

                     state <= `term_clear; // go to screen clear

                     wrtchr <= 0; // remove ready to write

                     outrdy <= 1; // set ready to send

                  end else if (chrdatw == 8'h1b) begin // escape

                     wrtchr <= 0; // remove ready to write

                     outrdy <= 1; // set ready to send

                     state <= `term_esc; // go escape handler

                  end else begin // unsupported control character

                     wrtchr <= 0; // remove ready to write

                     outrdy <= 1; // set ready to send

                  end

               end

            end

         end

         `term_wrtstd2: begin // write standard character #2

            cmwrite <= 1; // set write to memory

            state <= `term_wrtstd3; // continue

         end

         `term_wrtstd3: begin // write standard character #3

            cmwrite <= 0; // remove write to memory

            state <= `term_wrtstd4; // continue

         end

         `term_wrtstd4: begin // write standard character #3

            cmdatae <= 0; // release data enable to memory

            cmattre <= 0; // release attribute enable to memory

            outrdy <= 1; // set ready to send

            cursor <= cursor+1; // advance cursor

            wrtchr <= 0; // remove ready to write

            state <= `term_idle; // continue

         end

         `term_clear: begin // clear screen and home cursor

            cmaddr <= 0; // clear buffer address

            cmdatai <= 8'h20; // clear to spaces

            cmattri <= 5'b0; // clear attributes

            cmdatae <= 1; // enable data to memory

            cmattre <= 1; // enable attributes to memory

            state <= `term_clear2; // continue

         end

         `term_clear2: begin // clear screen and home cursor #2

            cmwrite <= 1; // set write to memory

            state <= `term_clear3; // continue

         end

         `term_clear3: begin // clear screen and home cursor #2

            cmwrite <= 0; // reset write to memory

            state <= `term_clear4; // continue

         end

         `term_clear4: begin // clear screen and home cursor #4

            if (cmaddr < `scnchrs*`scnlins) begin

               cmaddr <= cmaddr+1; // next character

               // Uncomment the next to put an incrementing pattern instead of

               // spaces.

               //cmdatai <= cmdatai+1;

               state <= `term_clear2; // continue

            end else begin // done

               outrdy <= 1; // set ready to send

               cursor <= 0; // set cursor to home position

               cmdatae <= 0; // release data enable to memory

               cmattre <= 0; // release attribute enable to memory

               state <= `term_idle; // continue

            end

         end

         `term_fndstr: begin // find start of current line

            if (tcursor+80 > cursor) begin // found

               cursor <= tcursor; // set cursor to line start

               wrtchr <= 0; // remove ready to write

               outrdy <= 1; // set ready to send

               state <= `term_idle; // continue

            end else tcursor <= tcursor+80; // advance to next line

         end

         `term_scroll: begin // scroll screen up

            // move all data up a line

            if (tcursor < 80*23) begin // scroll up

               cmread <= 1; // set read display

               cmaddr <= tcursor+80; // set address to read

               state <= `term_scroll1; // continue

            end else state <= `term_scroll5; // go blank last line

         end

         `term_scroll1: begin // scroll screen up #1

            state <= `term_scroll2; // hold read

         end

         `term_scroll2: begin // scroll screen up #2

            cmdatai <= cmdata; // get data at address

            cmattri <= cmattr; // get attribute at address

            cmread <= 0; // turn off read

            state <= `term_scroll3; // continue

         end

         `term_scroll3: begin // scroll screen up #3

            cmdatae <= 1; // enable data to write

            cmattre <= 1; // enable attribute to write

            cmwrite <= 1; // set to write

            cmaddr <= tcursor;

            state <= `term_scroll4; // continue

         end

         `term_scroll4: begin // scroll screen up #4

            cmwrite <= 0; // turn off write

            cmdatae <= 0; // turn off data enable

            cmattre <= 0; // turn off attribute enable

            tcursor <= tcursor+1; // next address

            state <= `term_scroll; // repeat character move

         end

         `term_scroll5: begin // scroll screen up #5

            // blank out last line

            if (tcursor < 80*24) begin // blank out

               cmdatai <= 8'h20; // set to write spaces

               cmattri <= 0; // set no attribute

               cmdatae <= 1; // enable data to write

               cmattre <= 1; // enable attribute to write

               cmwrite <= 1; // set to write

               cmaddr <= tcursor;

               state <= `term_scroll6; // continue

            end else begin // terminate

               wrtchr <= 0; // remove ready to write

               outrdy <= 1; // set ready to send

               state <= `term_idle; // continue

            end

         end

         `term_scroll6: begin // scroll screen up #6

            cmwrite <= 0; // turn off write

            cmdatae <= 0; // turn off data enable

            cmattre <= 0; // turn off attribute enable

            tcursor <= tcursor+1; // next address

            state <= `term_scroll5; // repeat blank out

         end

         `term_esc: begin // handle escape codes

            // wait for next character

            if (wrtchr&!(select&read|select&write)) begin

               // check its a cursor position, or "\esc="

               if (chrdatw == 8'h3d) begin

                  wrtchr <= 0; // remove ready to write

                  outrdy <= 1; // set ready to send

                  state <= `term_poscur;

               // check its a attribute set, or "\escG"

               end else if (chrdatw == 8'h47) begin

                  wrtchr <= 0; // remove ready to write

                  outrdy <= 1; // set ready to send

                  state <= `term_attset;

               end else begin // invalid sequence, abort

                  wrtchr <= 0; // remove ready to write

                  outrdy <= 1; // set ready to send

                  state <= `term_idle;

               end

            end

         end

         `term_poscur: begin // handle cursor direct position

            // wait for next character

            if (wrtchr&!(select&read|select&write)) begin

               rowchr <= chrdatw; // save row character

               wrtchr <= 0; // remove ready to write

               outrdy <= 1; // set ready to send

               state <= `term_poscur2;

            end

         end

         `term_poscur2: begin // handle cursor direct position #2

            // wait for next character

            if (wrtchr&!(select&read|select&write)) begin

               // check row and collumn are valid

               if (rowchr >= 8'h20 && rowchr <= 8'h37 &&

                   chrdatw >= 8'h20 && chrdatw <= 8'h6f)

                  // perform position

                  cursor <= (rowchr-8'h20)*80+(chrdatw-8'h20);

               wrtchr <= 0; // remove ready to write

               outrdy <= 1; // set ready to send

               state <= `term_idle;

            end

         end

         `term_attset: begin // handle attribute set

            // wait for next character

            if (wrtchr&!(select&read|select&write)) begin

               // Process attribute set code. The ADM 3A attributes are arranged

               // so that the bits in characters '0'-'N' correspond to attribute

               // bits. Some of the combinations are "invalid", but these are

               // ones that override each other. For example, blank overrides

               // all others.

               if (chrdatw >= 8'h30 && chrdatw <= 8'h4e) // its '0'-'N'

                  curatr <= chrdatw-8'h30 & 5'b11111;

               wrtchr <= 0; // remove ready to write

               outrdy <= 1; // set ready to send

               state <= `term_idle;

            end

         end

         default: state <= 6'bx;

      endcase

   end

   // Enable drive to character memory

   assign cmdata = cmdatae ? cmdatai: 8'bz;

   // Enable drive to character attributes

   assign cmattr = cmattre ? cmattri: 8'bz;

   // Enable drive for data output

   assign data = read&select ? datao: 8'bz;

endmodule

////////////////////////////////////////////////////////////////////////////////

//

// MEMORY MAPPED CHARACTER DISPLAY

//

// Contains the 20x8 characters used in a 640x480, 24 lines of 80 characters

// display. This display is accessed vi a 1920 character memory, which can be

// written and read. It is dual port, so has no restrictions on read and write.

//

// This block can either be directly mapped to CPU main memory, or can be run

// via a terminal emulator.

//

// Note there wasn't a mode for 24 lines, or for 640x480 pixels for alpha mode

// on the PC. Instead, 640x480 was a standard VGA mode for graphics. I like it

// because it unifies graphics and alpha modes, and has even divisions to

// character cells. Its a bit difficult to fit characters into a 8x20 cell,

// but the modes used for alpha, like 640x400 or 640x300 are not 4:3 aspect

// ratios, and really work by stretching the pixels vertically. We just do

// the same thing by explicit mapping.

//

`define clkfreq   50000000              // input clock frequency

`define blinkfreq 1                     // blink cycle in seconds

`define blinkmax  (`clkfreq*`blinkfreq) // total blink cycle

module chrmemmap(rst_n, clk, r, g, b, hsync_n, vsync_n, addr, read, write,

                 data, attr, cursor);

      input         rst_n;   // reset

      input         clk;     // master clock

      output [2:0]  r, g, b; // R,G,B color output buses

      output        hsync_n; // horizontal sync pulse

      output        vsync_n; // vertical sync pulse

      input  [10:0] addr;    // address to read or write

      input         read;    // read from address

      input         write;   // write from address

      inout  [7:0]  data;    // data to be written/read

      inout  [4:0]  attr;    // attributes to be written/read

      input [10:0]  cursor;  // cursor address

   reg [15:0] pixeldata; // 16 bit pixel feed

   reg  [6:0]  chrcnt;   // character counter

   reg  [4:0]  rowcnt;   // character row counter

   reg  [4:0]  lincnt;   // line counter

   reg  [10:0] scnadr;   // screen character buffer address

   reg  [7:0]  curchr;   // current character indexed by scnadr

   reg  [4:0]  curatr;   // current attribute indexed by scnadr

   reg  [1:0]  fchsta;   // character fetch state, 0 = load high, 1 = load low

   wire [10:0] chradr;   // character generator address

   wire [7:0]  chrdata;  // character generator data

   reg  [7:0]  datao;    // intermediate for data output

   reg  [4:0]  attro;    // intermediate for attribute output

   reg  [7:0]  pixdatl;  // pixel data low holding

   reg  [31:0] blinkcnt; // blink cycle counter

   reg         blon;     // blink on/off

   // storage for character based screen

   reg [7:0] scnbuf[1919:0]; // screen

   reg [4:0] atrbuf[1919:0]; // attributes

   assign rst = ~rst_n; // change reset polarity

   vga vgai(.rst(rst), .clk(clk), .pixel_data_in(pixeldata), .rd(rd), .eof(eof),

            .r(r), .g(g), .b(b), .hsync_n(hsync_n), .vsync_n(vsync_n),

            .blank(blank));

   chrrom crom(chradr, chrdata); // place character generator

   // run the blink cycle counter

   always @(posedge clk)

      if (rst) begin

      blinkcnt <= 0; // clear blink cycle

      blon <= 0; // clear cursor blink on

   end else begin

      blinkcnt <= blinkcnt+1; // count blink cycle

      // check blink cycle maxed, recycle if so

      if (blinkcnt >= `blinkmax) begin

         blinkcnt <= 0; // clear blink count

         blon <= ~blon; // flip blink state

      end

   end

   // run the character to screen scan

   always @(posedge clk)

      if (rst) begin // if reset

      // ????? SIMULATION PLUG

      // starting the character counter at the end of the line allows the scan

      // to cross the area the CPU is filling, and is ok at hardware time.

      // chrcnt <= 7'h0; // clear counters

      chrcnt <= 80-20; // clear counters

      // ????? SIMULATION PLUG

      // Starting the row count at 1 allows pixels to appear on the simulation,

      // and produces only a single bad line in real hardware

      rowcnt <= 5'h0;

      // rowcnt <= 5'h1;

      lincnt <= 5'h0;

      scnadr <= 11'h0;

      fchsta <= 0;

   end else if (eof) begin // if end of frame

      chrcnt <= 7'h0; // clear counters

      rowcnt <= 5'h0;

      lincnt <= 5'h0;

      scnadr <= 11'h0;

      fchsta <= 1; // set to fetch first set of characters

   end else if (rd || fchsta) begin

      if (fchsta == 1 || fchsta == 3) begin

         // advance counters

         if (chrcnt < 79) chrcnt <= chrcnt+1; // next character count

         else begin // next row

            chrcnt <= 0; // reset character

            if (rowcnt < 19) rowcnt <= rowcnt+1; // next character row

            else begin // next line

               rowcnt <= 0; // reset row

               lincnt <= lincnt+1; // next line

               scnadr <= scnadr+80; // advance character fetch

            end

         end

      end

      // Choose high or low character, and next state. Note we have to flip the

      // characters left to right to be correct.

      case (fchsta)

         0: fchsta <= 1; // delay until rd cycle is over

         1: begin

            // Set low bits of pixel register, and reverse if cursor matches the

            // current position, or if reverse attribute is on. Turn it all off

            // if blank is on.

            if ((scnadr+chrcnt == cursor)^curatr[2])

               pixdatl <= ~(curatr[0]|(curatr[1]&blon)? 8'h00: // blink and blank

                             (curatr[3]&rowcnt == 14? 8'hff: // underline

                              { chrdata[0], chrdata[1], chrdata[2], chrdata[3],

                                chrdata[4], chrdata[5], chrdata[6], chrdata[7] }));

            else

               pixdatl <= curatr[0]|(curatr[1]&blon)? 8'h00: // blink and blank

                             (curatr[3]&rowcnt == 14? 8'hff: // underline

                              { chrdata[0], chrdata[1], chrdata[2], chrdata[3],

                                chrdata[4], chrdata[5], chrdata[6], chrdata[7] });

            fchsta <= 2; // next state

         end

         2: fchsta <= 3; // delay a cycle for ROM read time

         3: begin

            // form 2 bit pixel data by doubling each character pixel

            pixeldata[0] <= chrdata[7];

            pixeldata[1] <= chrdata[7];

            pixeldata[2] <= chrdata[6];

            pixeldata[3] <= chrdata[6];

            pixeldata[4] <= chrdata[5];

            pixeldata[5] <= chrdata[5];

            pixeldata[6] <= chrdata[4];

            pixeldata[7] <= chrdata[4];

            pixeldata[8] <= chrdata[3];

            pixeldata[9] <= chrdata[3];

            pixeldata[10] <= chrdata[2];

            pixeldata[11] <= chrdata[2];

            pixeldata[12] <= chrdata[1];

            pixeldata[13] <= chrdata[1];

            pixeldata[14] <= chrdata[0];

            pixeldata[15] <= chrdata[0];

//            // Set low bits of pixel register, and reverse if cursor matches the

//            // current position, or if reverse attribute is on. Turn it all off

//            // if blank is on.

//            if ((scnadr+chrcnt == cursor)^curatr[2])

//               pixeldata <= pixdatl |

//                 (~(curatr[0]|(curatr[1]&blon)? 8'h00: // blink and blank

//                    (curatr[3]&rowcnt == 14? 8'hff: // underline

//                     { chrdata[0], chrdata[1], chrdata[2], chrdata[3],

//                       chrdata[4], chrdata[5], chrdata[6], chrdata[7] })) &

//                  8'hff) << 8;

//            else

//               pixeldata <= pixdatl |

//                 (curatr[0]|(curatr[1]&blon)? 8'h00: // blink and blank

//                    (curatr[3]&rowcnt == 14? 8'hff: // underline

//                     { chrdata[0], chrdata[1], chrdata[2], chrdata[3],

//                       chrdata[4], chrdata[5], chrdata[6], chrdata[7] })) << 8;

            fchsta <= 0; // back to start

         end

      endcase

   end

   // operate dual port screen character RAM

   always @(posedge clk) begin

      // set current indexed character without parity

      curchr <= scnbuf[scnadr+chrcnt] & 8'h7f;