diff --git a/bitmul.s b/bitmul.s
new file mode 100644
index 0000000..df84704
--- /dev/null
+++ b/bitmul.s
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+; the old 16-bit bit-and-shift multiplier
+; copied from old code, won't compile as-is
+
+; inner loop for imul16
+; bitnum < 8: 25 or 41 cycles
+; bitnum >= 8: 30 or 46 cycles
+.macro bitmul16 arg1, arg2, result, bitnum
+    .local zero
+    .local one
+    .local next
+
+    ; does 16-bit adds
+    ; arg1 and arg2 are treated as unsigned
+    ; negative signed inputs must be flipped first
+
+    ; 7 cycles up to the branch
+
+    ; check if arg1 has 0 or 1 bit in this place
+    ; 5 cycles either way
+    .if bitnum < 8
+        lda arg1                 ; 3 cyc
+        and #(1 << (bitnum))       ; 2 cyc
+    .else
+        lda arg1 + 1             ; 3 cyc
+        and #(1 << ((bitnum) - 8)) ; 2 cyc
+    .endif
+    bne one ; 2 cyc
+
+zero: ; 18 cyc, 23 cyc
+    lsr result + 3 ; 5 cyc
+    jmp next       ; 3 cyc
+
+one: ; 32 cyc, 37 cyc
+    ; 16-bit add on the top bits
+    clc            ; 2 cyc
+    lda result + 2 ; 3 cyc
+    adc arg2       ; 3 cyc
+    sta result + 2 ; 3 cyc
+    lda result + 3 ; 3 cyc
+    adc arg2 + 1   ; 3 cyc
+    ror a          ; 2 cyc - get a jump on the shift
+    sta result + 3 ; 3 cyc
+next:
+    ror result + 2 ; 5 cyc
+    ror result + 1 ; 5 cyc
+    .if bitnum >= 8
+        ; we can save 5 cycles * 8 bits = 40 cycles total by skipping this byte
+        ; when it's all uninitialized data
+        ror result ; 5 cyc
+    .endif
+
+.endmacro
+
+; 5 to 25 cycles
+.macro check_sign arg
+    ; Check sign bit and flip argument to postive,
+    ; keeping a count of sign bits in the X register.
+    .local positive
+    lda arg + 1   ; 3 cyc
+    bpl positive  ; 2 cyc
+    neg16 arg     ; 18 cyc
+    inx           ; 2 cyc
+positive:
+.endmacro
+
+; 518 - 828 cyc
+.macro imul16 dest, arg1, arg2
+    copy16 FR0, arg1  ; 12 cyc
+    copy16 FR1, arg2  ; 12 cyc
+    jsr imul16_func   ; 470-780 cyc
+    copy32 dest, FR2  ; 24 cyc
+.endmacro
+
+
+.macro shift_round_16 arg, shift
+    .repeat shift
+        shl32 arg
+    .endrepeat
+    round16 arg
+.endmacro
+
+.macro imul16_round dest, arg1, arg2, shift
+    copy16 FR0, arg1  ; 12 cyc
+    copy16 FR1, arg2  ; 12 cyc
+    jsr imul16_func   ; 470-780 cyc
+    shift_round_16 FR2, shift
+    copy16 dest, FR2 + 2  ; 12 cyc
+.endmacro
+
+; min 470 cycles
+; max 780 cycles
+.proc imul16_func
+    arg1 = FR0   ; 16-bit arg (clobbered)
+    arg2 = FR1   ; 16-bit arg (clobbered)
+    result = FR2 ; 32-bit result
+
+    ldx #0          ; 2 cyc
+    ; counts the number of sign bits in X
+    check_sign arg1 ; 5 to 25 cyc
+    check_sign arg2 ; 5 to 25 cyc
+    
+    ; zero out the 32-bit temp's top 16 bits
+    lda #0          ; 2 cyc
+    sta result + 2  ; 3 cyc
+    sta result + 3  ; 3 cyc
+    ; the bottom two bytes will get cleared by the shifts
+
+    ; unrolled loop for maximum speed, at the cost
+    ; of a larger routine
+    ; 440 to 696 cycles
+    .repeat 16, bitnum
+        ; bitnum < 8: 25 or 41 cycles
+        ; bitnum >= 8: 30 or 46 cycles
+        bitmul16 arg1, arg2, result, bitnum
+    .endrepeat
+
+    ; In case of mixed input signs, return a negative result.
+    cpx #1              ; 2 cyc
+    bne positive_result ; 2 cyc
+    neg32 result        ; 34 cyc
+positive_result:
+
+    rts ; 6 cyc
+.endproc