Marco Pistella wrote on 2026-05-02, 14:04:

For comparison purposes it would be very useful to repeat Command 4 on 800×600 and 1280×1024 with the original unmodified BIOS […]
Show full quote

For comparison purposes it would be very useful to repeat
Command 4 on 800×600 and 1280×1024 with the original
unmodified BIOS, if you still have access to it. That would confirm whether the jitter is BIOS-induced or
hardware-specific.

I have lost original bios. Not really looking forward to experiment with downloaded ones.
However I have similar PowerColor Radeon 9600 Pro (RV350) with orig BIOS. So I put it into same PC. Please note that I have CRT monitor - Samsung SyncMaster 765mb in case it plays any role.

Falcosoft wrote on 2026-05-03, 08:01:

@Marco Pistella: Do 64-bit reads/writes use FPU or MMX registers?
BTW, I very much like your test program 😀 I think this is the only available DOS tool that can measure such high bandwidths reliably.

Thank you — and glad you find it useful!

To answer your question: 64-bit reads and writes use FPU
registers via x87 (fild/fistp). MMX is intentionally not
used, to maintain compatibility with systems without MMX
support. The finit instruction can introduce significant
overhead on some CPUs, which may explain the 64-bit
anomaly you observed.

To investigate further: use the cursor keys to navigate
to the overhead screens in the benchmark results. These
show the routine overhead in PIT cycles for each access
width and allow a more accurate interpretation of the
raw numbers — particularly for the 64-bit case where
finit overhead may be the dominant factor.

vutt wrote on 2026-05-03, 09:13:

I have lost original bios. Not ... [CUT]

Thank you for the comparison with the original BIOS —
this is exactly the kind of controlled test that helps
isolate the cause of the anomalies seen earlier.

The pattern is irregular and the values are still high for a card
of this generation — which suggests the issue may
not be entirely BIOS-related.

To investigate further, could you press D on the
timing screen for each mode to display the horizontal
frequency sample distribution, and post the screenshots?
The shape of the distribution will help understand
whether the jitter has a specific pattern or is random
noise.

Marco Pistella wrote on 2026-05-03, 10:16:

To investigate further, could you press D on the timing screen for each mode to display the horizontal
frequency sample distribution, and post the screenshots?

As you can see it's different every time.

vutt wrote on 2026-05-03, 12:52:

... [CUT]

Thank you for the distribution screenshots.

The pattern is interesting: almost all samples cluster
at the expected PIT value, but a few outliers appear
significantly above — some exceeding 9,000 and one
above 12,000 PIT cycles. This is not random noise —
it is a discrete, sporadic event.

Could you press + to zoom in on the distribution chart
around the outlier area, and post a screenshot?
Specifically I would like to see whether the outliers
cluster around a specific value (for example a fixed
multiple of the main peak) or are scattered randomly.
This will help distinguish between a spurious bit 3
event on port 3DAh — where the measurement accidentally
captures two retrace intervals instead of one — and a
genuine timing instability in the video signal itself.

Marco Pistella wrote on 2026-05-03, 10:07:

.... To answer your question: 64-bit reads and writes use FPU registers via x87 (fild/fistp). MMX is intentionally not used, t […]
Show full quote

....
To answer your question: 64-bit reads and writes use FPU
registers via x87 (fild/fistp). MMX is intentionally not
used, to maintain compatibility with systems without MMX
support. The finit instruction can introduce significant
overhead on some CPUs, which may explain the 64-bit
anomaly you observed.

To investigate further: use the cursor keys to navigate
to the overhead screens in the benchmark results. These
show the routine overhead in PIT cycles for each access
width and allow a more accurate interpretation of the
raw numbers — particularly for the 64-bit case where
finit overhead may be the dominant factor.

Hi,
Here are the overhead results in PIT cycles. It seems they confirm your theory somewhat:

Falcosoft wrote on 2026-05-03, 16:28:

Hi,
Here...[CUT]

Thank you for the overhead data.

The FPU overhead values are within normal range — 19-23
PIT cycles for 64-bit and 128-bit reads, 19-20 for
writes. Nothing anomalous here that would explain the
64-bit performance deviation.

The cause remains unclear. With FPU overhead ruled out,
the anomaly may reflect a low-level interaction between
x87 transfer instructions and the memory subsystem on
this specific CPU/chipset combination — something that
does not manifest the same way on other architectures
tested so far. Possibly only an Intel engineer with
access to internal microarchitecture documentation
could give a definitive answer.

This is precisely the kind of unexpected discovery that
makes real hardware testing valuable.

For reference, here is the W_64L routine (64-bit linear
VRAM write via x87 FPU) that produces the anomalous
result on Ivy Bridge. The routine uses fst unrolled 8x
with fninit before the loop. If anyone has insight into
why 64-bit writes behave unexpectedly on this specific
microarchitecture, I would be very interested.

1;#############################################################################
2
3	W_064L:
4
5;#############################################################################
6
7	test	ds: byte ptr [status_flag_0],FLAG_FPU_PRESENT
8	jne	enter_fpu_w_64l
9	stc
10	ret
11enter_fpu_w_64l:
12	pusha
13	mov	cx,2h
14	call	Init_Align_Data16
15	mov	di,4h
16loop_w_064l:
17	mov	esi,ds: dword ptr [current_linear_vram_address]
18	mov	cx,4096d
19	fninit
20	fld	ds: qword ptr [bx]
21	xor	ax,ax
22	mov	ds,ax
23loopw_float_64l:
24	fst	ds: qword ptr [esi]
25	fst	ds: qword ptr [esi + 8h]
26	fst	ds: qword ptr [esi + 10h]
27	fst	ds: qword ptr [esi + 18h]
28	fst	ds: qword ptr [esi + 20h]
29	fst	ds: qword ptr [esi + 28h]
30	fst	ds: qword ptr [esi + 30h]
31	fst	ds: qword ptr [esi + 38h]
32	add	esi,40h
33	dec	cx
34	jne	loopw_float_64l
35	push	gs
36	pop	ds
37	mov	cx,2h
38	call	Inc_Aligned_Data16
39	dec	di
40	jne	loop_w_064l
41	add	ds: dword ptr [kb_proc],1024d
42	clc
43	popa
44	ret

Marco Pistella wrote on 2026-05-03, 17:23:

Thank you for the overhead data. […]
Show full quote

Falcosoft wrote on 2026-05-03, 16:28:

Hi,
Here...[CUT]

Thank you for the overhead data.

The FPU overhead values are within normal range — 19-23
PIT cycles for 64-bit and 128-bit reads, 19-20 for
writes. Nothing anomalous here that would explain the
64-bit performance deviation.

The cause remains unclear. With FPU overhead ruled out,
the anomaly may reflect a low-level interaction between
x87 transfer instructions and the memory subsystem on
this specific CPU/chipset combination — something that
does not manifest the same way on other architectures
tested so far. Possibly only an Intel engineer with
access to internal microarchitecture documentation
could give a definitive answer.

This is precisely the kind of unexpected discovery that
makes real hardware testing valuable.

To test if the results are Ivy Bridge specific I enabled the integrated GFX on my Haswell (i7-4770K 3500 MHz), and also enabled write-combining. It seems this anomaly is not Ivy Bridge specific and affects Haswell CPUs even more.
It seems on Haswell even SSE moves are affected somewhat:

The interesting thing is that this 64-bit FPU write anomaly only affects the write-combining enabled state. If write-combining is disabled the scaling is the expected one (but of course the absolute performance is much worse):

@Edit:
I have a theory: Since there are no larger bit depth than 32-bit supported in graphics modes the write-combining logic is more optimized for 8,16 and 32-bit writes. So larger than 32-bit writes somewhat interfere with the write combining logic that results in diminishing return.

Falcosoft wrote on 2026-05-03, 18:38:

I have a theory: Since there are ... [CUT]

That is a reasonable theory. The write-combining logic
in the memory controller is indeed optimized around the
native transaction sizes of the graphics subsystem —
32-bit being the natural boundary for legacy VGA and
VESA framebuffer access. Writes larger than 32-bit via
x87 or SSE may not map cleanly onto the write-combining
buffer granularity, causing partial fills or early
flushes that reduce the effectiveness of coalescing.

It would be consistent with what the data shows:
write-combining provides the expected benefit up to
32-bit, then the return diminishes at 64-bit and
partially recovers at 128-bit and 256-bit where the
transfer size aligns better with cache line boundaries.

To help confirm or rule out this theory, a few things
would be useful: the results page without overhead
subtraction to verify the raw values at these extreme
transfer rates, and multiple runs of the benchmark to
check whether the 64-bit write results are consistent
or show significant variation between runs. If the
values fluctuate widely it would suggest measurement
instability rather than a genuine microarchitecture
effect.

Confirming definitively would require access to Intel
memory controller documentation at a level of detail
that is not publicly available — possibly only an
Intel engineer could give a final answer.

Marco Pistella wrote on 2026-05-03, 19:18:

That is a reasonable theory. The write-combining logic in the memory controller is indeed optimized around the native transact […]
Show full quote

Falcosoft wrote on 2026-05-03, 18:38:

I have a theory: Since there are ... [CUT]

That is a reasonable theory. The write-combining logic
in the memory controller is indeed optimized around the
native transaction sizes of the graphics subsystem —
32-bit being the natural boundary for legacy VGA and
VESA framebuffer access. Writes larger than 32-bit via
x87 or SSE may not map cleanly onto the write-combining
buffer granularity, causing partial fills or early
flushes that reduce the effectiveness of coalescing.

It would be consistent with what the data shows:
write-combining provides the expected benefit up to
32-bit, then the return diminishes at 64-bit and
partially recovers at 128-bit and 256-bit where the
transfer size aligns better with cache line boundaries.

To help confirm or rule out this theory, a few things
would be useful: the results page without overhead
subtraction to verify the raw values at these extreme
transfer rates, and multiple runs of the benchmark to
check whether the 64-bit write results are consistent
or show significant variation between runs. If the
values fluctuate widely it would suggest measurement
instability rather than a genuine microarchitecture
effect.

Confirming definitively would require access to Intel
memory controller documentation at a level of detail
that is not publicly available — possibly only an
Intel engineer could give a final answer.

The results are very consistent: from 10 results the difference is about 1%!
The results without overhead are somewhat higher but the proportions are the same:

Falcosoft wrote on 2026-05-03, 19:27:

The results are very consistent: fr ... [CUT]

Thank you for the consistency data — 1% variation across
10 runs rules out measurement instability as a cause.

The results without overhead show the same proportions,
which also rules out FPU overhead as an explanation.

At this point the most honest conclusion is that this
is a previously undocumented behavior: write-combining
efficiency degrades specifically at 64-bit granularity
on Intel integrated graphics, recovers partially at
128-bit, and reaches its peak at 256-bit. The effect
is reproducible and consistent across Ivy Bridge and
Haswell microarchitectures.

Whether this is a deliberate design choice in the
memory controller, a side effect of the write-combining
buffer granularity, or something else entirely is beyond
what can be determined from these measurements alone.
Further data from other Intel platforms would be welcome,
and if anyone has deeper knowledge of Intel memory
controller internals, I would be very interested to hear
their interpretation.

Marco, do you have any idea why I can enable write combining on all Intel integrated graphics from Broadwell below (ie, 1st-5th gen), but my laptop systems always crash on 6th gen and above Intel integrated graphics when attempting to enable writecombining? Is it some kind of BIOS bug and / or castration or limitation on later intel core integrated graphics? I get a hard system freeze when attempting to enable writecombining with either Falcosoft's or RayerR's MTRRLFBE tools or even FastVid from 1996, and always must reboot on Skylake and above.

I'll post some results from i7-5600 Broadwell Intel integrated graphics and i5-8250 Kaby Lake Integrared graphics to show this. I also notice the 64-bit slump and recovery at 128 and max at 256b on Broadwell Intel Integrated graphics using write-combining and DDR3 dual-channel system RAM as reserved graphics memory. On the Kaby Lake system, writecombining wasn't possibe, but I of course got linear progression on uncombined writes but much slower than the writecombined Broadwell system (about 1/40th the speed).

X-VESA is by far the most useful and most comprehensive DOS VESA program I've ever seen. Thanks for releasing it and for all of your feedback.

zyzzle wrote on Yesterday, 07:36:

Marco, do you have any idea ... [CUT]

Thank you for the kind words.

Regarding the Skylake+ write-combining crash: I suspect
it may be related to a problem I encountered with my
AVX/AVX-512 routines in X-VESA. On one test system the
AVX routines were crashing after transferring between
170 and 280 KB of VRAM — meaning they worked, at least
briefly. The crashes disappeared completely when I
replaced the USB keyboard with a PS/2 one. The discovery
was entirely accidental.

My hypothesis is that USB legacy mode SMI handling
corrupts some CPU state during the critical section —
but this is a hypothesis, not a confirmed diagnosis.
It also seems to depend on the BIOS CSM/UEFI
implementation rather than the chipset itself.

If you can, try connecting a PS/2 keyboard and if
possible disabling USB legacy support in the BIOS
before attempting to enable write-combining. I have
no expectation that it will solve the problem, but
the result — whether it changes anything or not —
would help determine whether the two effects are
related or completely independent.

Marco Pistella wrote on Yesterday, 08:56:

Regarding the Skylake+ write-combining crash: I suspect it may be related to a problem I encountered with my AVX/AVX-512 routi […]
Show full quote

Regarding the Skylake+ write-combining crash: I suspect
it may be related to a problem I encountered with my
AVX/AVX-512 routines in X-VESA. On one test system the
AVX routines were crashing after transferring between
170 and 280 KB of VRAM — meaning they worked, at least
briefly. The crashes disappeared completely when I
replaced the USB keyboard with a PS/2 one. The discovery
was entirely accidental.

My hypothesis is that USB legacy mode SMI handling
corrupts some CPU state during the critical section —
but this is a hypothesis, not a confirmed diagnosis.
It also seems to depend on the BIOS CSM/UEFI
implementation rather than the chipset itself.

Interesting theory. Your behavior mirrors mine. Sometimes MTRRs seem to be enabled for a very brief time (ie, program prints LFB writecombining is enabled just before complete system freeze. It may be BIOS/CSM specific, but I've tested about 15 different brand laptops with Intel Integrated GPUs and they all fail to enable MTRRs on Skylake and above, but will on Broadwell. As to UEFI implementation, I have tried a mix of UEFI implentations on different brands of laptops for Broadwell and below, and they all work fine. Even the same brand laptop with a Skylake vs a Broadwell chip (and same UEFI) freezes on the 6th gen chip while enables MTRRs on the 5th gen. Only thing changed is the Intel onboard vBIOS.

Unfortunately I can't attach PS/2 keyboard to any of these systems, as they're all laptops, using the builtin keyboards. None has any extant PS/2 ports. I can attach a USB keyboard which will disable the builtin keyboard, however; I'll see if that has any different behavior. I suspect some sort of "legacy mode" implementation bug. Disabling USB Legacy mode if possible will disable my keyboard input, however.

It will help greatly if anyone may test Skylake and above Intel core systems, with Intel integrated onboard graphics in baremetal DOS, and report if writecombining can be successfully enabled. Report your system stats (CPU and exact brand / model of laptop or system used.)

zyzzle wrote on Yesterday, 22:50:

Interesting theory. Your behavior mirrors mine. Sometimes MTRRs seem to be enabled for a very brief time (ie, program prints LFB […]
Show full quote

Marco Pistella wrote on Yesterday, 08:56:

Regarding the Skylake+ write-combining crash: I suspect it may be related to a problem I encountered with my AVX/AVX-512 routi […]
Show full quote

Regarding the Skylake+ write-combining crash: I suspect
it may be related to a problem I encountered with my
AVX/AVX-512 routines in X-VESA. On one test system the
AVX routines were crashing after transferring between
170 and 280 KB of VRAM — meaning they worked, at least
briefly. The crashes disappeared completely when I
replaced the USB keyboard with a PS/2 one. The discovery
was entirely accidental.

My hypothesis is that USB legacy mode SMI handling
corrupts some CPU state during the critical section —
but this is a hypothesis, not a confirmed diagnosis.
It also seems to depend on the BIOS CSM/UEFI
implementation rather than the chipset itself.

Interesting theory. Your behavior mirrors mine. Sometimes MTRRs seem to be enabled for a very brief time (ie, program prints LFB writecombining is enabled just before complete system freeze. It may be BIOS/CSM specific, but I've tested about 15 different brand laptops with Intel Integrated GPUs and they all fail to enable MTRRs on Skylake and above, but will on Broadwell. As to UEFI implementation, I have tried a mix of UEFI implentations on different brands of laptops for Broadwell and below, and they all work fine. Even the same brand laptop with a Skylake vs a Broadwell chip (and same UEFI) freezes on the 6th gen chip while enables MTRRs on the 5th gen. Only thing changed is the Intel onboard vBIOS.

Unfortunately I can't attach PS/2 keyboard to any of these systems, as they're all laptops, using the builtin keyboards. None has any extant PS/2 ports. I can attach a USB keyboard which will disable the builtin keyboard, however; I'll see if that has any different behavior. I suspect some sort of "legacy mode" implementation bug. Disabling USB Legacy mode if possible will disable my keyboard input, however.

It will help greatly if anyone may test Skylake and above Intel core systems, with Intel integrated onboard graphics in baremetal DOS, and report if writecombining can be successfully enabled. Report your system stats (CPU and exact brand / model of laptop or system used.)

Hi,
Just to check whether write-combining mode itself or rather the linear frame buffer is the problem on your systems try this little tool:

It only checks and sets the write-combining mode of the real mode VGA color graphics frame buffer (A0000 - AFFFF).
It does this step by step so if it fails we can see at least at what step it fails. If it runs successfully then you can see that the default 'Window - via Int 10h' writes speed up significantly in X-VESA. This means that it can speed up any, even high resolution VESA 1.2 or VESA 2+ modes when linear frame buffer is not used.
Unfortunately I have no Haswell+ system to test this, but here are my Ivy Bridge results:

VGA frame buffer (A0000 - AFFFF) Write combining disabled:

VGA frame buffer (A0000 - AFFFF) Write combining enabled: