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[/] [forwardcom/] [trunk/] [call_stack.sv] - Blame information for rev 13

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1 13 Agner
//////////////////////////////////////////////////////////////////////////////////
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// Engineer: Agner Fog
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//
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// Create Date:       2020-05-05
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// Last modified:     2021-07-27
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// Module Name:       call_stack
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// Project Name:      ForwardCom soft core
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// Target Devices:    Artix 7
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// Tool Versions:     Vivado v. 2020.1
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// License:           CERN-OHL-W v. 2 or later
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// Description:       on-chip call stack. controls call and return instructions
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//
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//////////////////////////////////////////////////////////////////////////////////
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`include "defines.vh"
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// call stack, 1024*32 bits, used for function return addresses but not for parameters or local data
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// On call instruction: put return address on push_data and set call_e high for one clock cycle.
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// On return instruction: the return address is pre-loaded into pop_data for early read.
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// Set return_e high for one clock cycle to make next return address ready.
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// It is not allowed to have call_e and return_e in the same clock cycle.
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// It is not allowed to have return_e in the first clock cycle after call_e
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// or return_e. This is not a problem in the current design because consecutive
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// jump, call, and return instructions are delayed by the access time to the code cache.
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// If the design is improved with more efficient branch prediction then it may be
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// necessary to add precautions to prevent a return_e signal in the first clock cycle
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// after another return_e or call_e.
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module call_stack (
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    input clock,                                 // clock
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    input clock_enable,                          // clock enable. Used when single-stepping
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    input reset,                                 // clock enable. Used when single-stepping
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    input call_e,                                // Executing call instruction. push_data contains return address
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    input return_e,                              // Executing return instruction. return address is available in advance on pop_data
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    input  [`CODE_ADDR_WIDTH-1:0] push_data,     // Return address pushed here at call instruction
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    output reg [`CODE_ADDR_WIDTH-1:0] pop_data,  // Return address popped here at return instruction
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    output reg overflow                          // stack overflow or underflow or error
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   );
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// site of stack = 2**stack_pointer_bits
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parameter integer stack_pointer_bits = `CALL_STACK_POINTER_BITS;
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// stack ram, on-chip
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reg [`CODE_ADDR_WIDTH-1:0] ram[0:(2**stack_pointer_bits)-1];
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// call stack pointer
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reg [stack_pointer_bits-1:0] call_stack_pointer = 0;
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// call and return operations = push and pop of function return addresses:
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always_ff @(posedge clock) if (clock_enable) begin
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    overflow <= 0;
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    if (reset) begin
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        call_stack_pointer <= 0;
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        pop_data <= 0;
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        overflow <= 0;
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    end else if (call_e) begin
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        if (call_stack_pointer == (2**stack_pointer_bits)-1 || return_e) begin
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            overflow <= 1;                          // overflow or other error
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        end else begin
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            ram[call_stack_pointer] <= push_data;   // push return address on stack
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            pop_data <= push_data;                  // make next return address ready
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            call_stack_pointer <= call_stack_pointer + 1;
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        end
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    end else if (return_e) begin
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        if (call_stack_pointer == 0) begin
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            overflow <= 1;                          // stack underflow
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        end else if (call_stack_pointer == 1) begin
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            pop_data <= 0;                          // stack empty
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            call_stack_pointer <= call_stack_pointer - 1;
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        end else begin
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            pop_data <= ram[call_stack_pointer-2];  // make next return address ready
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            call_stack_pointer <= call_stack_pointer - 1;
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        end
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    end
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end
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endmodule

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