summaryrefslogtreecommitdiff
path: root/apps/dsp_cf.S
blob: 497b551be38db883222e5512e692e135fbb940e7 (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
/***************************************************************************
 *             __________               __   ___.
 *   Open      \______   \ ____   ____ |  | _\_ |__   _______  ___
 *   Source     |       _//  _ \_/ ___\|  |/ /| __ \ /  _ \  \/  /
 *   Jukebox    |    |   (  <_> )  \___|    < | \_\ (  <_> > <  <
 *   Firmware   |____|_  /\____/ \___  >__|_ \|___  /\____/__/\_ \
 *                     \/            \/     \/    \/            \/
 * $Id$
 *
 * Copyright (C) 2006 Thom Johansen
 * Portions Copyright (C) 2007 Michael Sevakis
 *
 * All files in this archive are subject to the GNU General Public License.
 * See the file COPYING in the source tree root for full license agreement.
 *
 * This software is distributed on an "AS IS" basis, WITHOUT WARRANTY OF ANY
 * KIND, either express or implied.
 *
 ****************************************************************************/

/****************************************************************************
 * void apply_crossfeed(int count, int32_t *src[])
 */
    .section    .text
    .global     apply_crossfeed 
apply_crossfeed:
    lea.l       -44(%sp), %sp
    movem.l     %d2-%d7/%a2-%a6, (%sp)  | save all regs
    movem.l     48(%sp), %d7/%a4        | %d7 = count, %a4 = src
    movem.l     (%a4), %a4-%a5          | %a4 = src[0], %a5 = src[1]
    lea.l       crossfeed_data, %a1
    move.l      (%a1)+, %a6             | a6 = direct gain
    movem.l     12(%a1), %d0-%d3        | fetch filter history samples
    move.l      132(%a1), %a0           | fetch delay line address
    movem.l     (%a1), %a1-%a3          | load filter coefs
    /* Register usage in loop:
     * %a0 = delay_p, %a1..%a3 = b0, b1, a1 (filter coefs),
     * %a4 = src[0], %a5 = src[1], %a6 = direct gain,
     * %d0..%d3 = history
     * %d4..%d6 = temp.
     * %d7 = count
     */
.cfloop:
    mac.l       %a2, %d0, 4(%a0), %d0, %acc0 | acc  = b1*dr[n - 1] d0 = dr[n]
    mac.l       %a1, %d0             , %acc0 | acc += b0*dr[n]
    mac.l       %a3, %d1,  (%a4), %d4, %acc0 | acc += a1*y_l[n - 1], load L
    move.l      %acc0, %d1              | get filtered delayed sample
    mac.l       %a6, %d4, %acc0         | acc += gain*x_l[n]
    movclr.l    %acc0, %d6              |
    move.l      %d6, (%a4)+             | write result

    mac.l       %a2, %d2, (%a0), %d2, %acc0 | acc  = b1*dl[n - 1], d2 = dl[n]
    mac.l       %a1, %d2            , %acc0 | acc += b0*dl[n]
    mac.l       %a3, %d3, (%a5), %d5, %acc0 | acc += a1*y_r[n - 1], load R
    movem.l     %d4-%d5, (%a0)          | save left & right inputs to delay line
    move.l      %acc0, %d3              | get filtered delayed sample
    mac.l       %a6, %d5, %acc0         | acc += gain*x_r[n]
    lea.l       8(%a0), %a0             | increment delay pointer
    movclr.l    %acc0, %d6              |
    move.l      %d6, (%a5)+             | write result

    cmpa.l      #crossfeed_data+136, %a0| wrap a0 if passed end
    bge.b       .cfwrap                 |
    .word       0x51fb                  | tpf.l - trap the buffer wrap
.cfwrap:
    lea.l       -104(%a0), %a0          | wrap
    subq.l      #1, %d7                 | --count < 0 ?
    bgt.b       .cfloop                 |
    lea.l       crossfeed_data+16, %a1  | save data back to struct
    movem.l     %d0-%d3, (%a1)          | ...history
    move.l      %a0, 120(%a1)           | ...delay_p
    movem.l     (%sp), %d2-%d7/%a2-%a6  | restore all regs
    lea.l       44(%sp), %sp
    rts
.cfend:
    .size       apply_crossfeed,.cfend-apply_crossfeed


/****************************************************************************
 * int dsp_downsample(int count, struct dsp_data *data,
 *                    in32_t *src[], int32_t *dst[])
 */
    .section    .text
    .global     dsp_downsample
dsp_downsample:
    lea.l       -40(%sp), %sp           | save non-clobberables
    movem.l     %d2-%d7/%a2-%a5, (%sp)  |
    movem.l     44(%sp), %d2/%a0-%a2    | %d2 = count
                                        | %a0 = data
                                        | %a1 = src
                                        | %a2 = dst
    movem.l     4(%a0), %d3-%d4         | %d3 = ch = data->num_channels
                                        | %d4 = delta = data->resample_data.delta
    moveq.l     #16, %d7                | %d7 = shift
.dschannel_loop:
    move.l      12(%a0), %d5            | %d5 = phase = data->resample_data.phase
    move.l      -4(%a1, %d3.l*4), %a3   | %a3 = s = src[ch-1]
    move.l      -4(%a2, %d3.l*4), %a4   | %a4 = d = dst[ch-1]
    lea.l       12(%a0, %d3.l*4), %a5   | %a5 = &data->resample_data.ast_sample[ch-1]
    move.l      (%a5), %d0              | %d0 = last = data->resample_data.last_sample[ch-1]
    move.l      -4(%a3, %d2.l*4), (%a5) | data->resample_data.last_sample[ch-1] = s[count-1]
    move.l      %d5, %d6                | %d6 = pos = phase >> 16
    lsr.l       %d7, %d6                |
    cmp.l       %d2, %d6                | past end of samples?
    bge.b       .dsloop_skip            | yes? skip loop
    tst.l       %d6                     | need last sample of prev. frame?
    bne.b       .dsloop                 | no? start main loop
    move.l      (%a3, %d6.l*4), %d1     | %d1 = s[pos]
    bra.b       .dsuse_last_start       | start with last (last in %d0)
.dsloop:
    lea.l       -4(%a3, %d6.l*4), %a5   | load s[pos-1] and s[pos]
    movem.l     (%a5), %d0-%d1          |
.dsuse_last_start:
    sub.l       %d0, %d1                | %d1 = diff = s[pos] - s[pos-1]
    move.l      %d0, %acc0              | %acc0 = previous sample
    move.l      %d5, %d0                | frac = (phase << 16) >> 1
    lsl.l       %d7, %d0                |
    lsr.l       #1, %d0                 |
    mac.l       %d0, %d1, %acc0         | %acc0 += frac * diff
    add.l       %d4, %d5                | phase += delta
    move.l      %d5, %d6                | pos = phase >> 16
    lsr.l       %d7, %d6                |
    movclr.l    %acc0, %d0              |
    move.l      %d0, (%a4)+             | *d++ = %d0
    cmp.l       %d2, %d6                | pos < count?
    blt.b       .dsloop                 | yes? continue resampling
.dsloop_skip:
    subq.l      #1, %d3                 | ch > 0?
    bgt.b       .dschannel_loop         | yes? process next channel
    asl.l       %d7, %d2                | wrap phase to start of next frame
    sub.l       %d2, %d5                | data->resample_data.phase =
    move.l      %d5, 12(%a0)            | ... phase - (count << 16)
    move.l      %a4, %d0                | return d - d[0]
    sub.l       (%a2), %d0              |
    asr.l       #2, %d0                 | convert bytes->samples
    movem.l     (%sp), %d2-%d7/%a2-%a5  | restore non-clobberables
    lea.l       40(%sp), %sp            | cleanup stack
    rts                                 | buh-bye
.dsend:
    .size       dsp_downsample,.dsend-dsp_downsample

/****************************************************************************
 * int dsp_upsample(int count, struct dsp_data *dsp,
 *                  in32_t *src[], int32_t *dst[])
 */
    .section    .text
    .global     dsp_upsample
dsp_upsample:
    lea.l       -40(%sp), %sp           | save non-clobberables
    movem.l     %d2-%d7/%a2-%a5, (%sp)  |
    movem.l     44(%sp), %d2/%a0-%a2    | %d2 = count
                                        | %a0 = data
                                        | %a1 = src
                                        | %a2 = dst
    movem.l      4(%a0), %d3-%d4        | %d3 = ch = channels
                                        | %d4 = delta = data->resample_data.delta
    swap        %d4                     | swap delta to high word to use
                                        | carries to increment position
.uschannel_loop:
    move.l      12(%a0), %d5            | %d5 = phase = data->resample_data.phase
    move.l      -4(%a1, %d3.l*4), %a3   | %a3 = s = src[ch-1]
    lea.l       12(%a0, %d3.l*4), %a4   | %a4 = &data->resample_data.last_sample[ch-1]
    lea.l       (%a3, %d2.l*4), %a5     | %a5 = src_end = &src[count]
    move.l      (%a4), %d0              | %d0 = last = data->resample_data.last_sample[ch-1]
    move.l      -(%a5), (%a4)           | data->resample_data.last_sample[ch-1] = s[count-1]
    move.l      -4(%a2, %d3.l*4), %a4   | %a4 = d = dst[ch-1]
    swap        %d5                     | swap phase to high word to use
                                        | carries to increment position
    move.l      %d5, %d6                | %d6 = pos = phase >> 16
    clr.w       %d5                     |
    eor.l       %d5, %d6                | pos == 0?
    beq.b       .usstart_0              | no? transistion from down
    cmp.l       %d3, %d6                | past end of samples?
    bge.b       .usloop_skip            | yes? skip loop
    lea.l       -4(%a3, %d6.l*4), %a3   | %a3 = s = &s[pos-1] (previous)
    move.l      (%a3)+, %d0             | %d0 = *s++
    .word       0x51fa                  | tpf.w - trap next instruction
.usloop_1:
    move.l      %d6, %d0                | move previous sample to %d0
.usstart_0:
    move.l      (%a3)+, %d1             | fetch next sample
    move.l      %d1, %d6                | save sample value
    sub.l       %d0, %d1                | %d1 = diff = s[pos] - s[pos-1]
.usloop_0:
    lsr.l       #1, %d5                 | make phase into frac
    mac.l       %d1, %d5, %acc0         | %acc0 = diff * frac
    lsl.l       #1, %d5                 | restore frac to phase
    movclr.l    %acc0, %d7              | %d7 = product
    add.l       %d0, %d7                | %d7 = last + product
    move.l      %d7, (%a4)+             | *d++ = %d7
    add.l       %d4, %d5                | phase += delta
    bcc.b       .usloop_0               | load next values?
    cmp.l       %a5, %a3                | src <= src_end?
    ble.b       .usloop_1               | yes? continue resampling
.usloop_skip:
    subq.l      #1, %d3                 | ch > 0?
    bgt.b       .uschannel_loop         | yes? process next channel
    swap        %d5                     | wrap phase to start of next frame
    move.l      %d5, 12(%a0)            | ...and save in data->resample_data.phase
    move.l      %a4, %d0                | return d - d[0]
    sub.l       (%a2), %d0              |
    movem.l     (%sp), %d2-%d7/%a2-%a5  | restore non-clobberables
    asr.l       #2, %d0                 | convert bytes->samples
    lea.l       40(%sp), %sp            | cleanup stack
    rts                                 | buh-bye
.usend:
    .size       dsp_upsample,.usend-dsp_upsample

/* These routines might benefit from burst transfers but we'll keep them
 * small for now since they're rather light weight
 */

/****************************************************************************
 * void channels_process_sound_chan_mono(int count, int32_t *buf[])
 *
 * Mix left and right channels 50/50 into a center channel.
 */
    .section    .text
    .global     channels_process_sound_chan_mono
channels_process_sound_chan_mono:
    movem.l     4(%sp), %d0/%a0         | %d0 = count, %a0 = buf
    lea.l       -12(%sp), %sp           | save registers
    move.l      %macsr, %d1             |
    movem.l     %d1-%d3, (%sp)          |
    move.l      #0xb0, %macsr           | put emac in rounding fractional mode
    movem.l     (%a0), %a0-%a1          | get channel pointers
    move.l      #0x40000000, %d3        | %d3 = 0.5
1:
    move.l     (%a0), %d1               | L = R = l/2 + r/2
    mac.l      %d1, %d3, (%a1), %d2, %acc0 |
    mac.l      %d2, %d3, %acc0          |
    movclr.l   %acc0, %d1               |
    move.l     %d1, (%a0)+              | output to original buffer
    move.l     %d1, (%a1)+              |
    subq.l     #1, %d0                  |
    bgt.s      1b                       |
    movem.l    (%sp), %d1-%d3           | restore registers
    move.l     %d1, %macsr              |
    lea.l      12(%sp), %sp             | cleanup
    rts
.cpmono_end:
    .size       channels_process_sound_chan_mono, .cpmono_end-channels_process_sound_chan_mono


/****************************************************************************
 * void channels_process_sound_chan_custom(int count, int32_t *buf[])
 *
 * Apply stereo width (narrowing/expanding) effect.
 */
    .section    .text
    .global     channels_process_sound_chan_custom
channels_process_sound_chan_custom:
    movem.l     4(%sp), %d0/%a0         | %d0 = count, %a0 = buf
    lea.l       -16(%sp), %sp           | save registers
    move.l      %macsr, %d1             |
    movem.l     %d1-%d4, (%sp)          |
    move.l      #0xb0, %macsr           | put emac in rounding fractional mode
    movem.l     (%a0), %a0-%a1          | get channel pointers
    move.l      dsp_sw_gain, %d3        | load straight (mid) gain
    move.l      dsp_sw_cross, %d4       | load cross (side) gain
1:
    move.l      (%a0), %d1              |
    mac.l       %d1, %d3, (%a1), %d2, %acc0 |  L = l*gain + r*cross
    mac.l       %d1, %d4            , %acc1 |  R = r*gain + l*cross
    mac.l       %d2, %d4            , %acc0 |
    mac.l       %d2, %d3            , %acc1 |
    movclr.l    %acc0, %d1              |
    movclr.l    %acc1, %d2              |
    move.l      %d1, (%a0)+             |
    move.l      %d2, (%a1)+             |
    subq.l      #1, %d0                 |
    bgt.s       1b                      |
    movem.l     (%sp), %d1-%d4          | restore registers
    move.l      %d1, %macsr             |
    lea.l       16(%sp), %sp            | cleanup
    rts
.cpcustom_end:
    .size       channels_process_sound_chan_custom, .cpcustom_end-channels_process_sound_chan_custom

/****************************************************************************
 *  void channels_process_sound_chan_karaoke(int count, int32_t *buf[])
 *
 *  Separate channels into side channels.
 */
    .section    .text
    .global     channels_process_sound_chan_karaoke
channels_process_sound_chan_karaoke:
    movem.l     4(%sp), %d0/%a0         | %d0 = count, %a0 = buf
    lea.l       -16(%sp), %sp           | save registers
    move.l      %macsr, %d1             |
    movem.l     %d1-%d4, (%sp)          |
    move.l      #0xb0, %macsr           | put emac in rounding fractional mode
    movem.l     (%a0), %a0-%a1          | get channel pointers
    move.l      #0x40000000, %d4        | %d3 = 0.5
1:
    move.l     (%a0), %d1               |
    msac.l     %d1, %d4, (%a1), %d2, %acc0 | R = r/2 - l/2
    mac.l      %d2, %d4            , %acc0 |
    movclr.l   %acc0, %d1               |
    move.l     %d1, (%a1)+              |
    neg.l      %d1                      | L = -R = -(r/2 - l/2) = l/2 - r/2
    move.l     %d1, (%a0)+              |
    subq.l     #1, %d0                  |
    bgt.s      1b                       |
    movem.l    (%sp), %d1-%d4           | restore registers
    move.l     %d1, %macsr              |
    lea.l      16(%sp), %sp             | cleanup
    rts
.cpkaraoke_end:
    .size       channels_process_sound_chan_karaoke, .cpkaraoke_end-channels_process_sound_chan_karaoke
/****************************************************************************
 * void sample_output_stereo(int count, struct dsp_data *data,
 *                               int32_t *src[], int16_t *dst)
 *
 * Framework based on the ubiquitous Rockbox line transfer logic for
 * Coldfire CPUs.
 *
 * Does emac clamping and scaling (which proved faster than the usual
 * checks and branches - even single test clamping) and writes using
 * line burst transfers. Also better than writing a single L-R pair per
 * loop but a good deal more code.
 *
 * Attemping bursting during reads is rather futile since the source and
 * destination alignments rarely agree and too much complication will
 * slow us up. The parallel loads seem to do a bit better at least until
 * a pcm buffer can always give line aligned chunk and then aligning the
 * dest can then imply the source is aligned if the source buffers are.
 * For now longword alignment is assumed of both the source and dest.
 *
 */
    .section   .text
    .global    sample_output_stereo
sample_output_stereo:
    lea.l       -44(%sp), %sp             | save registers
    move.l      %macsr, %d1               | do it now as at many lines will
    movem.l     %d1-%d7/%a2-%a5, (%sp)    | be the far more common condition
    move.l      #0x80, %macsr             | put emac unit in signed int mode
    movem.l     48(%sp), %a0-%a2/%a4      |
    lea.l       (%a4, %a0.l*4), %a0       | %a0 = end address     
    move.l      (%a1), %d1                | %a1 = multiplier: (1 << (16 - scale))
    sub.l       #16, %d1                  |
    neg.l       %d1                       |
    moveq.l     #1, %d0                   |
    asl.l       %d1, %d0                  |
    move.l      %d0, %a1                  |
    movem.l     (%a2), %a2-%a3            | get L/R channel pointers
    moveq.l     #28, %d0                  | %d0 = second line bound
    add.l       %a4, %d0                  |
    and.l       #0xfffffff0, %d0          |
    cmp.l       %a4, %d0                  | at least a full line?
    blo.w       .sos_longloop_1_start     | no? jump to trailing longword
    sub.l       #16, %d0                  | %d1 = first line bound
    cmp.l       %a4, %d0                  | any leading longwords?
    bls.b       .sos_lineloop_start       | no? jump to line loop
.sos_longloop_0:
    move.l      (%a2)+, %d1               | read longword from L and R
    mac.l       %d1, %a1, (%a3)+, %d2, %acc0 | shift L to high word
    mac.l       %d2, %a1, %acc1           | shift R to high word
    movclr.l    %acc0, %d1                | get possibly saturated results
    movclr.l    %acc1, %d2                |
    swap        %d2                       | move R to low word
    move.w      %d2, %d1                  | interleave MS 16 bits of each 
    move.l      %d1, (%a4)+               | ...and write both
    cmp.l       %a4, %d0                  |
    bhi.b       .sos_longloop_0           |
.sos_lineloop_start:
    lea.l       -12(%a0), %a5             | %a5 = at or just before last line bound
.sos_lineloop:
    move.l      (%a3)+, %d4               | get next 4 R samples and scale
    mac.l       %d4, %a1, (%a3)+, %d5, %acc0 | with saturation
    mac.l       %d5, %a1, (%a3)+, %d6, %acc1 |
    mac.l       %d6, %a1, (%a3)+, %d7, %acc2 |
    mac.l       %d7, %a1, (%a2)+, %d0, %acc3 |
    lea.l       16(%a4), %a4              | increment dest here, mitigate stalls
    movclr.l    %acc0, %d4                | obtain R results
    movclr.l    %acc1, %d5                |
    movclr.l    %acc2, %d6                |
    movclr.l    %acc3, %d7                |
    mac.l       %d0, %a1, (%a2)+, %d1, %acc0 | get next 4 L samples and scale
    mac.l       %d1, %a1, (%a2)+, %d2, %acc1 | with saturation
    mac.l       %d2, %a1, (%a2)+, %d3, %acc2 |
    mac.l       %d3, %a1             , %acc3 |
    swap        %d4                       | a) interleave most significant...
    swap        %d5                       |
    swap        %d6                       |
    swap        %d7                       |
    movclr.l    %acc0, %d0                | obtain L results
    movclr.l    %acc1, %d1                |
    movclr.l    %acc2, %d2                |
    movclr.l    %acc3, %d3                |
    move.w      %d4, %d0                  | a) ... 16 bits of L and R
    move.w      %d5, %d1                  |
    move.w      %d6, %d2                  |
    move.w      %d7, %d3                  |
    movem.l     %d0-%d3, -16(%a4)         | write four stereo samples
    cmp.l       %a4, %a5                  |
    bhi.b       .sos_lineloop             |
.sos_longloop_1_start:
    cmp.l       %a4, %a0                  | any longwords left?
    bls.b       .sos_done                 | no? finished.
.sos_longloop_1:
    move.l      (%a2)+, %d1               | handle trailing longwords
    mac.l       %d1, %a1, (%a3)+, %d2, %acc0 | the same way as leading ones
    mac.l       %d2, %a1, %acc1           |
    movclr.l    %acc0, %d1                |
    movclr.l    %acc1, %d2                |
    swap        %d2                       |
    move.w      %d2, %d1                  |
    move.l      %d1, (%a4)+               |
    cmp.l       %a4, %a0                  |
    bhi.b       .sos_longloop_1           |
.sos_done:
    movem.l     (%sp), %d1-%d7/%a2-%a5    | restore registers
    move.l      %d1, %macsr               |
    lea.l       44(%sp), %sp              | cleanup
    rts                                   |
.sos_end:
    .size      sample_output_stereo, .sos_end-sample_output_stereo

/****************************************************************************
 * void sample_output_mono(int count, struct dsp_data *data,
 *                         int32_t *src[], int16_t *dst)
 *
 * Same treatment as sample_output_stereo but for one channel.
 */
    .section   .text
    .global    sample_output_mono
sample_output_mono:
    lea.l       -28(%sp), %sp             | save registers
    move.l      %macsr, %d1               | do it now as at many lines will
    movem.l     %d1-%d5/%a2-%a3, (%sp)    | be the far more common condition
    move.l      #0x80, %macsr             | put emac unit in signed int mode
    movem.l     32(%sp), %a0-%a3          |
    lea.l       (%a3, %a0.l*4), %a0       | %a0 = end address     
    move.l      (%a1), %d1                | %d5 = multiplier: (1 << (16 - scale))
    sub.l       #16, %d1                  |
    neg.l       %d1                       |
    moveq.l     #1, %d5                   |
    asl.l       %d1, %d5                  |
    movem.l     (%a2), %a2                | get source channel pointer
    moveq.l     #28, %d0                  | %d0 = second line bound
    add.l       %a3, %d0                  |
    and.l       #0xfffffff0, %d0          |
    cmp.l       %a3, %d0                  | at least a full line?
    blo.w       .som_longloop_1_start     | no? jump to trailing longword
    sub.l       #16, %d0                  | %d1 = first line bound
    cmp.l       %a3, %d0                  | any leading longwords?
    bls.b       .som_lineloop_start       | no? jump to line loop
.som_longloop_0:
    move.l      (%a2)+, %d1               | read longword from L and R
    mac.l       %d1, %d5, %acc0           | shift L to high word
    movclr.l    %acc0, %d1                | get possibly saturated results
    move.l      %d1, %d2                  |
    swap        %d2                       | move R to low word
    move.w      %d2, %d1                  | duplicate single channel into
    move.l      %d1, (%a3)+               | L and R
    cmp.l       %a3, %d0                  |
    bhi.b       .som_longloop_0           |
.som_lineloop_start:
    lea.l       -12(%a0), %a1             | %a1 = at or just before last line bound
.som_lineloop:
    move.l      (%a2)+, %d0               | get next 4 L samples and scale
    mac.l       %d0, %d5, (%a2)+, %d1, %acc0 | with saturation
    mac.l       %d1, %d5, (%a2)+, %d2, %acc1 |
    mac.l       %d2, %d5, (%a2)+, %d3, %acc2 |
    mac.l       %d3, %d5             , %acc3 |
    lea.l       16(%a3), %a3              | increment dest here, mitigate stalls
    movclr.l    %acc0, %d0                | obtain results
    movclr.l    %acc1, %d1                |
    movclr.l    %acc2, %d2                |
    movclr.l    %acc3, %d3                |
    move.l      %d0, %d4                  | duplicate single channel
    swap        %d4                       | into L and R
    move.w      %d4, %d0                  |
    move.l      %d1, %d4                  |
    swap        %d4                       |
    move.w      %d4, %d1                  |
    move.l      %d2, %d4                  |
    swap        %d4                       |
    move.w      %d4, %d2                  |
    move.l      %d3, %d4                  |
    swap        %d4                       |
    move.w      %d4, %d3                  |
    movem.l     %d0-%d3, -16(%a3)         | write four stereo samples
    cmp.l       %a3, %a1                  |
    bhi.b       .som_lineloop             |
.som_longloop_1_start:
    cmp.l       %a3, %a0                  | any longwords left?
    bls.b       .som_done                 | no? finished.
.som_longloop_1:
    move.l      (%a2)+, %d1               | handle trailing longwords
    mac.l       %d1, %d5, %acc0           | the same way as leading ones
    movclr.l    %acc0, %d1                |
    move.l      %d1, %d2                  |
    swap        %d2                       |
    move.w      %d2, %d1                  |
    move.l      %d1, (%a3)+               |
    cmp.l       %a3, %a0                  |
    bhi.b       .som_longloop_1           |
.som_done:
    movem.l     (%sp), %d1-%d5/%a2-%a3    | restore registers
    move.l      %d1, %macsr               |
    lea.l       28(%sp), %sp              | cleanup
    rts                                   |
.som_end:
    .size      sample_output_mono, .som_end-sample_output_mono