Stack
The Stack module implements a dedicated hardware stack for the B32P2 CPU, providing push and pop operations for function calls and local variables. The stack is designed as a 32-bit wide, 1024-word deep memory structure. There currently is no way to access either the stack pointer or data that is not at the stack pointer directly. Also, there is no overflow or underflow protection, so the stack pointer can wrap around and access invalid memory locations. The stack support hold and clear signals to allow pipeline control.
Module Declaration
module Stack (
input wire clk, // System clock
input wire reset, // Reset signal
input wire [31:0] d, // Data to push onto stack
output wire [31:0] q, // Data popped from stack
input wire push, // Push operation enable
input wire pop, // Pop operation enable
input wire clear, // Clear/flush signal
input wire hold // Hold/stall signal
);
Stack Architecture
Memory Organization
- Width: 32 bits per stack entry
- Depth: 1024 words (4KiB total)
- Addressing: Internal 10-bit stack pointer
- Growth direction: Upward (incrementing addresses)
Stack Operations
Push Operation
When push is asserted:
- Store data
dat current stack pointer location - Increment stack pointer
- Debug output (simulation only)
Pop Operation
Pop operation is more complex due to pipeline integration:
if (pop) begin
useRamResult <= 1'b0;
ramResult <= stack[ptr - 1'b1]; // Read data from stack
if (clear) begin
qreg <= 32'd0; // Flush: return zero
end else if (hold) begin
qreg <= qreg; // Stall: maintain current value
end else begin
useRamResult <= 1'b1; // Normal: use stack data
ptr <= ptr - 1'b1; // Decrement stack pointer
`ifdef __ICARUS__ // Debug output if run in simulation
$display("%d: pop ptr %d := %d", $time, ptr, stack[ptr - 1'b1]);
`endif
end
end
Memory Characteristics
The stack array is designed to infer as Block RAM: