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Address Translator (Arithmetic)

This is the run-time adjustable version of the Static Address Translator.

Translates a variable, arbitrary, unaligned range of N locations into an aligned range (starting at zero, up to N-1) so the translated address can be used to sequentially index into other addressed components (multiplexers, address decoders, RAMs, etc...).

When memory-mapping a small memory or a number of control registers to a base address that isn't a power-of-2, the Least Significant Bits (LSBs) which vary over the address range will not address the mapped entries in order. This addressing offset scrambles the order of the control registers and of the memory locations so that the mapped order no longer matches the physical order of the actual hardware, which makes design and debugging harder.

If the address range does not start at a power-of-2 boundary, the LSBs will not count in strictly increasing order and will not start at zero: the order of the numbers the LSBs represent is rotated by the offset to the nearest power-of-2 boundary. Also, if the address range does not span a power-of-2 sized block, then not all possible LSB values will be present.

However, we can add the offset back to translate the raw LSBs into consecutive LSBs to then directly address the memory or control registers. A separate Address Decoder signals when the translation is valid.

For example, take 4 locations at addresses 7 to 10, which we would like to map to a range of 0 to 3. We want address 7 to access the zeroth location, and so on. To map 4 locations, the address' two LSBs must be translated as follows:

0111 (7)  --> 0100 (4)
1000 (8)  --> 1001 (5)
1001 (9)  --> 1010 (6)
1010 (10) --> 1011 (7)

You can see the raw two LSBs are in the sequence 3,0,1,2, which we must map to 0,1,2,3. This means re-aligning the addresses to the nearest power-of-2 boundary, leaving the LSBs in the order we need, and that's a simple subtraction of the value of the initial, untranslated LSBs (3).

Typically, you'll need this Address Translator alongside an Address Decoder module which decodes a variable address range: either the Behavioural or Arithmetic decoders.

Since we only translate the LSBs, the higher address bits are unused here, and would trigger CAD tool warnings about module inputs not driving anything. Thus, we don't select the LSBs from the whole address here, and it is up to the enclosing module to feed the Address Translator only the necessary LSBs, which is fine since the enclosing module must already know the number of LSBs to translate. The output is the translated LSBs in consecutive order.

`default_nettype none

module Address_Translator_Arithmetic
#(
    parameter       OUTPUT_ADDR_WIDTH   = 0
)
(
    input   wire    [OUTPUT_ADDR_WIDTH-1:0] offset,
    input   wire    [OUTPUT_ADDR_WIDTH-1:0] input_addr,
    output  wire    [OUTPUT_ADDR_WIDTH-1:0] output_addr
);

    Adder_Subtractor_Binary
    #(
        .WORD_WIDTH (OUTPUT_ADDR_WIDTH)
    )
    address_offset
    (
        .add_sub    (1'b1),    // 0/1 -> A+B/A-B
        .carry_in   (1'b0),
        .A_in       (input_addr),
        .B_in       (offset),
        .sum_out    (output_addr),
        // verilator lint_off PINCONNECTEMPTY
        .carry_out  ()
        // verilator lint_on  PINCONNECTEMPTY
    );

endmodule

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