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

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