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stand: Document that boot0 uses BIOS

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And thus has a limited range of supported baud rates. Also add that
setting BOOT_BOOT0_COMCONSOLE_SPEED=0 will leave it unchanged which
sometimes can give you 115200 if the BIOS initialized things outside of
the normal BIOS baud rates (which many x86 enbedded-targetted boards
do).

Sponsored by:		Netflix
Reviewed by:		emaste, manu (earlier versions)
Suggestions by:		jhb
Differential Revision:	https://reviews.freebsd.org/D36300
This commit is contained in:
Warner Losh 2022-08-26 15:47:21 -06:00
parent df065f699f
commit 7ed3228323
1 changed files with 7 additions and 0 deletions
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7
stand/i386/boot0/Makefile
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@ -42,6 +42,13 @@ BOOT_BOOT0_ORG?= 0x600
ORG=${BOOT_BOOT0_ORG}
# Comm settings for boot0sio.
#
# boot0sio uses BIOS INT $0x14 for serial ports, we can only support these
# baudrates due to INT14's limited interface. In addition, if
# BOOT_BOOT0_COMCONSOLE_SPEED=0, then the baud rate and frame format will remain
# unchanged. Some BIOSes initialize the serial ports to 115200, and this may
# allow boot0sio access at that rate if so.
#
# Bit(s) Description
# 7-5 data rate (110,150,300,600,1200,2400,4800,9600 bps)
# 4-3 parity (00 or 10 = none, 01 = odd, 11 = even)