Смутно помнится, что с 43 порта можно было снять время точнее.

Да!? А мне говорили, что по средам из Гринвича.

MasterZivАга, именно на 43 порт он прямо и передается непосредственно из института метрологии в Петербурге.
Не поленился достать из закромов DOS help:

HELP6 Ports 40H-43H control the PC timer circuits. There are three timer
channels:

0) Channel 0 is used as a general system timer. It handles the periodic
"timer-tick". It is initialized by the BIOS to invoke INT 08H
(IRQ 0) 18.206 times per second (once every 54.936 milliseconds).

1) Channel 1 is used in a dedicated manner to support the memory refresh
function. The output is used to generate a direct-memory access
(DMA) cycle request on DMA channel 0. This DMA channel is used to
refresh the systems RAM by performing a dummy memory-read every so
often (once every 15 microseconds on the original PC).

This is critical to system operation, so we are usually advised to
keep "hands off". However, I have seen experiments in which this
timer was slowed down a bit in order to increase overall system
performance by a few percent. Too slow of a value eventually causes
fatal memory errors.

2) Channel 2 is used to drive the audio speaker on the system board (it
originally handled cassette-tape I/O, but that is surely moot). The
output of channel 2 can be directed to control the speaker via I/O
port 61H (see PPI Ports and Speaker Control).

Channel 2 is available for application-controlled timing tasks;
however, it is a global resource. Rather than using direct port I/O
you are advised to use fns such as INT 1aH and INT 15H 86H for timed
operations.

Port Description
ЯЯЯЯ ЯЯЯЯЯЯЯЯЯЯЯЯЯЯЯЯЯЯЯЯЯЯЯЯЯЯЯЯЯЯЯЯЯЯЯЯЯЯЯЯЯЯЯЯЯЯЯЯЯЯЯЯЯЯЯЯЯЯЯЯЯЯЯЯЯЯЯЯЯЯ
40H Read/Write: Count value for counter 0
Port 43H bits 4-5 control the way the counter is set.
41H Read/Write: Count value for counter 1
41H Read/Write: Count value for counter 2
ДДДД ДДДДДДДДДДДДДДДДДДДДДДДДДДДДДДДДДДДДДДДДДДДДДДДДДДДДДДДДДДДДДДДДДДДДДДД
43H Write: set channel's mode of operation
Ц7В6В5В4В3В2В1В0·
єch#іr/lіmode і є
УДБДБДБДБДБДБДБТЅ bits mask
АТЩ АТЩ АДТДЩ ИН 0: 01H 0=process count as binary
є є є 1=process counts as BCD
є є ИННННН1-3: 0eH select timer mode:
є є 000 = mode 0: interrupt on terminal count
є є 001 = mode 1: programmable one-shot
є є x10 = mode 2: rate generator
є є x11 = mode 3: square-wave rate generator
є є 100 = mode 4: software-triggered strobe
є є 101 = mode 5: hardware-triggered strobe
є ИНННННННННН4-5: 30H select read/load sequence:
є 00 = latch counter for stable read
є 01 = read/load most significant byte only
є 10 = read/load least significant byte only
є 11 = read/load LSB then MSB
ИНННННННННННННН6-7: c0H specify counter to affect:
00 = counter 0, 01= counter 1
10 = counter 2, 11= counter 3

See Also: Speaker Control
PPI Ports
I/O Port Map
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Иногда полезно придержать сарказм - может вернуться.