Snakes!

The first game on my FPGA platform

This is a follow-up of the FPGA computer post. 

I have decided to make a game for the FPGA Computer. My friend gave me his Pascal implementation of the Snakes game and I have ported it into the FPGA Assembler. Not an easy task, but it works now:

The game was made to work in the video text mode. The frame is constructed using '-', '+', and '|' characters. The head of the snake is '@', the body is 'O', the star is '*', and when the snake hits the wall or its tail, we get the 'X' character at the crash scene.

In the emulator it works the same way:

The game is placed on the github.

Milliseconds counter register

During the game development, it occured to me that I need to implement the milliseconds counter register in order to implement the delay function. In the cpu.v file, I have created two additional registers:

reg [N-1:0] millis_counter;

reg [15:0] clock_counter;

The millis_counter register will hold the number of milliseconds counted so far. Incrementing this register is done whenever clock_counter reaches 50000 (clock is 50MHz, which means that when the clock_counter reaches 50000, one millisecond has elapsed):

always @ (posedge CLOCK_50) begin

if (clock_counter < 50000) begin

clock_counter <= clock_counter + 1'b1;

end

else begin

clock_counter <= 0;

millis_counter <= millis_counter + 1'b1;

end

...

To read the millis_counter register, I have introduced another port address for the IN instruction:

4'b0011: begin

  // IN reg, [xx]

  `ifdef DEBUG

  $display("%2x: IN r%-d, [%4d]",ir[3:0], (ir[11:8]), data);

  `endif

  case (mc_count)

0: begin

mbr <= data;  // remember the address of IO port

mc_count <= 1;

pc <= pc + 2'd2;  // move to the next instruction

end

1: begin

    case (mbr)

64: begin    // UART RX DATA

regs[ir[11:8]] <= {8'b0, rx_data_r};

end

65: begin   // UART TX BUSY

regs[ir[11:8]] <= tx_busy;

end

68: begin    // keyboard data

regs[ir[11:8]] <= {8'b0, ps2_data_r};

end

69: begin // milliseconds counted so far

regs[ir[11:8]] <= millis_counter;

end

  endcase // end of case(mbr)

  ir <= 0;      // initiate fetch

  addr <= pc >> 1;

end

default: begin

end

  endcase  // end of case (mc_count)

end // end of IN reg, [xx]

The example of the usage can be found in the snakes.asm file:

; ################################################################

; function delay(r0)

; waits for the r0 milliseconds

; ################################################################

delay:

push r1

push r2

delay_loop2:

in r1, [PORT_MILLIS] ; port 69

delay_loop1:

in r2, [PORT_MILLIS] ; port 69

sub r2, r1

jz delay_loop1 ; one millisecond elapsed here

dec r0

jnz delay_loop2

pop r2

pop r1

ret

Conclusion

This was the first game made for the FPGA Computer. The game is quite simple and uses video text mode. I had to implement a milliseconds counter register and the corresponding IN port to read it. It was used to implement the delay function.