Re: Tonights Tradeoff - Carry and Overflow

On Sat, 12 Oct 2024 9:38:01 +0000, Robert Finch wrote:

On 2024-10-09 6:44 a.m., Robert Finch wrote:
Mulled over carry and overflow in arithmetic operations. Looked at
widening the datapath to 66-bits to hold carry and overflow bits.
Thinking it may increase the size of the design by over 3% just to
support carry and overflow. For now, an instruction, ADDGC, was added to
generate the carry bit as a result. A 256-bit add looks like:
>
; 256 bit add
; A = r1,r2,r3,r4
; B = r5,r6,r7,r8
; S = r9,r10,r11,r12
>
add r9,r1,r5,r0
addgc r13,r1,r5,r0
add r10,r2,r6,r13
addgc r13,r2,r6,r13
add r11,r7,r3,r13
addgc r13,r7,r3,r13
add r12,r8,r4,r13

My 66000 version::
       CARRY   R8,{{IO}{IO}{IO}{O}}
       ADD     R4,R12,R16
       ADD     R5,R13,R17
       ADD     R6,R14,R18
       ADD     R7,R15,R19
            // R{8,7,6,5,4} contain the 257-bit result.
256-bit add giving 257-bit result.

>
Not very elegant a solution, but it is simple. I think it requires
minimal hardware. Three input ADD is already present and ADDGC just
routes the carry bit to the output.