TSConf_MiST/src/cpu/zint.v

module zint
(
  input wire clk,
  input wire zpos,
  input wire res,
  input wire int_start_frm,
  input wire int_start_lin,
  input wire int_start_dma,
  input wire vdos,
  input wire intack,
  input wire [7:0] intmask,
  output reg [7:0] im2vect,
  output wire int_n
);

// In VDOS INTs are focibly disabled.
// For Frame, Line INT its generation is blocked, it will be lost.
// For DMA INT only its output is blocked, so DMA ISR will will be processed as soon as returned from VDOS.

assign int_n = ~(int_frm || int_lin || int_dma) | vdos;

wire dis_int_frm = !intmask[0];
wire dis_int_lin = !intmask[1];
wire dis_int_dma = !intmask[2];

wire intack_s = intack && !intack_r;
reg intack_r;
always @(posedge clk) intack_r <= intack;

reg [1:0] int_sel;
always @(posedge clk) begin
	if (intack_s) begin
			  if (int_frm) im2vect <= 8'hFF;    // priority 0
		else if (int_lin) im2vect <= 8'hFD;    // priority 1
		else if (int_dma) im2vect <= 8'hFB;    // priority 2
	end
end

// ~INT generating
reg int_frm;
always @(posedge clk) begin
		  if (res || dis_int_frm)     int_frm <= 0;
	else if (int_start_frm)          int_frm <= 1;
	else if (intack_s || intctr_fin) int_frm <= 0;   // priority 0
end

reg int_lin;
always @(posedge clk) begin
		  if (res || dis_int_lin)   int_lin <= 0;
	else if (int_start_lin)        int_lin <= 1;
	else if (intack_s && !int_frm) int_lin <= 0;   // priority 1
end

reg int_dma;
always @(posedge clk) begin
		  if (res || dis_int_dma)               int_dma <= 0;
	else if (int_start_dma)                    int_dma <= 1;
	else if (intack_s && !int_frm && !int_lin) int_dma <= 0; // priority 2
end

// ~INT counter
reg [5:0] intctr;
wire intctr_fin = intctr[4];   // 32 clks

always @(posedge clk, posedge int_start_frm) begin
	if (int_start_frm) intctr <= 0;
	else if (zpos && !intctr_fin && !vdos) intctr <= intctr + 1'b1;
end

endmodule