# Phase 2: polymem -> BRAM banks

## Goal
Replace the single multi-port async-read `reg [11:0] polymem [0:28*256-1]` with
registered-read `sd_bram` banks that infer real BRAM (ASIC: compiled SRAM).

## Bank split (all sd_bram, W=12, 1R+1W registered)
- bank_a : A_hat[i][j], slots 0..K^2-1   -> D=4096 (KMAX^2*256), A=12
- bank_se: s_hat[i] then e_hat[i]         -> D=2048 (2*KMAX*256), A=11
            relative slot = abs_slot - slot_s_rt   (s: 0..K-1, e: K..2K-1)
- bank_t : t_hat[i], slots slot_t_rt..    -> D=1024 (KMAX*256), A=10
            relative slot = abs_slot - slot_t_rt

## Port budget (verified disjoint)
- poly_mul LOAD vs ACCUMULATE never overlap; ST_N/ST_M/ST_E are disjoint
  top-states. => each bank needs only 1R+1W. sd_bram fits.
- bank_t ST_M-acc reads t_hat[idx] (j>0 source) and writes t_hat[idx] result
  same cycle: 1R+1W, read-old/write-new. Prefetch keeps read AHEAD of write
  (monotonic idx), so no same-pass RAW hazard.

## Read sites (all need 1-cycle read-ahead vs async today)
- ntt_in       (ST_N load)      <- bank_se[n_slot]
- e_c0,e_c1    (ST_E)           <- bank_se (dk) or bank_t (ek); SERIALIZE the
                                   two reads across the 3-byte/pair window
- pm_a_in      (ST_M load)      <- bank_a[m_aslot]
- pm_b_in      (ST_M load)      <- bank_se[m_j]
- m_acc_src    (ST_M acc)       <- bank_se[K+m_i] (j==0) or bank_t[m_i] (j>0)
- dbg_coeff_o  (not on KAT path) <- route by slot range (compile-only)

## Write sites
- ST_A: bank_a[a_slot]            <= snt_coeff
- ST_C: bank_se[c_slot-slot_s]    <= cbd_modq
- ST_N: bank_se[n_slot]           <= ntt_coeff   (write-back, same slot)
- ST_M-acc: bank_t[m_i]           <= m_accq

## Read-ahead strategy
Cores hold ready_o high through entire LOAD (no mid-stream backpressure) =>
fixed 1-cycle skew. Pattern: advance a read-address pointer 1 cycle ahead of
the consumer index; delay the consumer's valid by 1 cycle ("prime the pipe").
For ST_M-acc (irregular pm_vo cadence 0,1,1): 2-entry prefetch skid buffer,
read pointer runs monotonically ahead of write pointer.

## Checkpoints (KAT-gated: 11 cases, byte-exact, 0 file-not-found)
- 2a: split polymem -> 4 ASYNC-read banks (pure refactor, zero timing change).
      Validates bank sizing + base-relative addressing + debug mux. COMMIT.
- 2b: convert banks to registered sd_bram + add read-ahead pipelines to every
      consumer FSM (ST_N, ST_E, ST_M load, ST_M acc). COMMIT.