• https://cdn.openbsd.org/pub/OpenBSD/7.5/arm64/install75.img

% ls -l  /Volumes/BOOT
total 11878
-rwx------  1 alexlai  staff    26894 Mar 21 03:10 bcm2710-rpi-2-b.dtb
-rwx------  1 alexlai  staff    29011 Mar 21 03:10 bcm2710-rpi-3-b-plus.dtb
-rwx------  1 alexlai  staff    28392 Mar 21 03:10 bcm2710-rpi-3-b.dtb
-rwx------  1 alexlai  staff    26890 Mar 21 03:10 bcm2710-rpi-cm3.dtb
-rwx------  1 alexlai  staff    49090 Mar 21 03:10 bcm2711-rpi-4-b.dtb
-rwx------  1 alexlai  staff    48810 Mar 21 03:10 bcm2711-rpi-400.dtb
-rwx------  1 alexlai  staff    49202 Mar 21 03:10 bcm2711-rpi-cm4.dtb
-rwx------  1 alexlai  staff    52456 Mar 21 03:10 bootcode.bin
-rwx------@ 1 alexlai  staff       65 Mar 21 03:10 config.txt
drwx------  1 alexlai  staff      512 Mar 21 03:10 efi
-rwx------  1 alexlai  staff     7314 Mar 21 03:10 fixup.dat
-rwx------  1 alexlai  staff     5448 Mar 21 03:10 fixup4.dat
drwx------  1 alexlai  staff      512 Mar 21 03:10 overlays
-rwx------  1 alexlai  staff  2953312 Mar 21 03:10 start.elf
-rwx------  1 alexlai  staff  2229120 Mar 21 03:10 start4.elf
-rwx------  1 alexlai  staff   570680 Mar 21 03:10 u-boot.bin

alexlai@MacMini Downloads % cat /Volumes/BOOT/config.txt 
arm_64bit=1
enable_uart=1
dtoverlay=disable-bt
kernel=u-boot.bin

alexlai@MacMini Downloads % echo "program_usb_boot_mode=1" >> /Volumes/BOOT/config.txt
alexlai@MacMini Downloads % cat /Volumes/BOOT/config.txt                              
arm_64bit=1
enable_uart=1
dtoverlay=disable-bt
kernel=u-boot.bin
program_usb_boot_mode=1

• https://cdn.openbsd.org/pub/OpenBSD/7.5/arm64/miniroot75.img

alexlai@MacMini Downloads % ls -l  /Volumes/BOOT                                      
total 11878
-rwx------  1 alexlai  staff    26894 Mar 21 02:06 bcm2710-rpi-2-b.dtb
-rwx------  1 alexlai  staff    29011 Mar 21 02:06 bcm2710-rpi-3-b-plus.dtb
-rwx------  1 alexlai  staff    28392 Mar 21 02:06 bcm2710-rpi-3-b.dtb
-rwx------  1 alexlai  staff    26890 Mar 21 02:06 bcm2710-rpi-cm3.dtb
-rwx------  1 alexlai  staff    49090 Mar 21 02:06 bcm2711-rpi-4-b.dtb
-rwx------  1 alexlai  staff    48810 Mar 21 02:06 bcm2711-rpi-400.dtb
-rwx------  1 alexlai  staff    49202 Mar 21 02:06 bcm2711-rpi-cm4.dtb
-rwx------  1 alexlai  staff    52456 Mar 21 02:06 bootcode.bin
-rwx------@ 1 alexlai  staff       65 Mar 21 02:06 config.txt
drwx------  1 alexlai  staff      512 Mar 21 02:06 efi
-rwx------  1 alexlai  staff     7314 Mar 21 02:06 fixup.dat
-rwx------  1 alexlai  staff     5448 Mar 21 02:06 fixup4.dat
drwx------  1 alexlai  staff      512 Mar 21 02:06 overlays
-rwx------  1 alexlai  staff  2953312 Mar 21 02:06 start.elf
-rwx------  1 alexlai  staff  2229120 Mar 21 02:06 start4.elf
-rwx------  1 alexlai  staff   570680 Mar 21 02:06 u-boot.bin

alexlai@MacMini Downloads % cat /Volumes/BOOT/config.txt
arm_64bit=1
enable_uart=1
dtoverlay=disable-bt
kernel=u-boot.bin
alexlai@MacMini Downloads % echo "program_usb_boot_mode=1" >> /Volumes/BOOT/config.txt
alexlai@MacMini Downloads % cat /Volumes/BOOT/config.txt                              
arm_64bit=1
enable_uart=1
dtoverlay=disable-bt
kernel=u-boot.bin
program_usb_boot_mode=1