The code at 7d8a is an interslot call to the subrom.
Below is my code which clearly shows NYYRIKKI is much better at this kind of stuff 
; ; Change bios to call the 'skip slot 1 or 2' in the subrom ; fpos 07d8ah rst 030h defb 083h ; It shouldn't hurt to hardcode the subrom slot defw 03f0eh ; Address of test_ab_key in subrom nop nop
Then this is the code in the subrom:
; ; Hold A or B key during boot to skip slot 1 or 2 respectively ; fpos 03f0eh test_ab_key: push bc ; c has slot push af ld a,c and 00000011b ; remove subslot ( just skip all secundary slots) ld c,a ;FBE7 (keyboard row 2): ;A - bf - 1011 1111 ;B - 7f - 0111 1111 ld a,(newkey+2) xor 255 and 11000000b ; we are only interested in bit 7+6 rla rla rla ; a should be 1 for 'A' and 2 for 'B' now cp c jr nz,test_ab_notpressed pop af ld c,a ; save a register or 1 ; reset z flag ld a,c ; restore a register pop bc ret test_ab_notpressed: pop af pop bc push hl ld hl,04241h rst 020h ; DCOMPR, compare HL with DE pop hl ret ; ret z if 4142 is found
I use sjasmplus to assemble this. I just do incbin of the original rom then use fpos to overwrite the code on the correct place in the original rom.
Key "1" would conflict with Nextor, so I'd need to use other keys.
This is exactly the reason I chose A and B instead of 1 and 2.
Ok... In this case I'll give you another tip as well...
During early boot MSX checks that the upper 32KB of RAM does not have bit errors and there is indeed 32KB (and not ie. 8KB or 16KB) ... It actually does this two times!... As all MSX2+ machines have minimum of 64KB of memory this is utterly useless check. Even if you have failed RAM-chips in your machine, I've newer heard that this would have actually saved the day for anyone.... Only thing it can do is to move BASIC programs storage area up from #8000 and this way reduce RAM in use and probably only make the real problem even more hard to pin point.
Personally I changed it so that it checks only every 256th byte, but I see no reason why the check could not be removed completely if you want... This will save few seconds of boot time and will make the RETURN-check much more seamless (as you don't need to hold it down while MSX is doing these checks)
Thanks NYRRIKKI, this is a great tip! I will try to integrate this in my rom.
Also it would be nice to merge all this stuff with the msxnewbios project and put it on github.
And wasn't there another project similar to msxnewbios? I can't seem to find it a the moment.
Thanks for your help,
Personally I changed it so that it checks only every 256th byte,
Could you tell me what BIOS function needs to be modded to achieve this? (name of the function or where is located?)
You can see these in #451 and #4B4
But I don't understand how your code works 
You got the disassembly correct, so I'll try to add some comments:
; NYYRIKKI's code disassembly
7fef cd8c01 call 018ch ; add a CALL to empty space before original INIT-routine
018c 79 ld a,c
e603 and 03h
c8 ret z ; In case of slot 0 return to normal INIT
fe03 cp 03h
c8 ret z ; In case of slot 3 return to normal INIT
c5 push bc
87 add a,a ; Check bits 1 & 2
47 ld b,a
af xor a ; ... on row 0
cdec0b call 0bech ;Subroutine SNSMAT Returns the value of the specified line from the keyboard matrix
f3 di
a0 and b ; compare the keyboard result to current slot number
c1 pop bc
c0 ret nz ; ... if not key down, return to normal INIT
c1 pop bc ; else pop the INIT address from stack (= do not execute INIT)
c9 ret
and Philip's code is also different but I don't understand where it does the magic:
7d8c 0e3f ld c,3fh ;address 03f -> You got this wrong way around... it is #3F0E
I only now realized that we practically answered at a same time.
Key "1" would conflict with Nextor, so I'd need to use other keys.
This is exactly the reason I chose A and B instead of 1 and 2.
Please enlighten me... I know that in MSX tR "1"-key is used to force the machine in to MSX-DOS1 mode, but is this same implemented now in Nextor as well?
Also it would be nice to merge all this stuff with the msxnewbios project and put it on github.
And wasn't there another project similar to msxnewbios? I can't seem to find it a the moment.
I must say I'm not familiar to msxnewbios project, but I've found msxsyssrc project quite priceless.
Thank you NYYRIKKI and Philip!! 
Then this is the code in the subrom:
Hahaha you are right NYYRIKKI, I had the address in the SubROM wrong! 
Please enlighten me... I know that in MSX tR "1"-key is used to force the machine in to MSX-DOS1 mode, but is this same implemented now in Nextor as well?
Yes, from Konamiman's page:
The Nextor kernel will boot in MSX-DOS 1 mode when the computer has no mapped RAM, when the "1" key is pressed at boot time, or when a MSX-DOS 1 boot sector is detected in the boot device.
Ok... In this case I'll give you another tip as well...
During early boot MSX checks that the upper 32KB of RAM does not have bit errors and there is indeed 32KB (and not ie. 8KB or 16KB) ... It actually does this two times!... As all MSX2+ machines have minimum of 64KB of memory this is utterly useless check. Even if you have failed RAM-chips in your machine, I've newer heard that this would have actually saved the day for anyone.... Only thing it can do is to move BASIC programs storage area up from #8000 and this way reduce RAM in use and probably only make the real problem even more hard to pin point.
Personally I changed it so that it checks only every 256th byte, but I see no reason why the check could not be removed completely if you want... This will save few seconds of boot time and will make the RETURN-check much more seamless (as you don't need to hold it down while MSX is doing these checks)
So finally I got around to try this.
The first check I changed so it only checks one byte for page 2 and one for page 3.
And the second check to find the bottom for basic programs I just hardcoded to 8000.
Then I did a little test with openmsx: let openmsx execute until right after the 2nd check and then exit.
The results with the normal memory check:
$ time openmsx -machine 8245_2p -script e.tcl real 0m1.435s user 0m0.233s sys 0m0.100s
And with the quick memory check:
$ time openmsx -machine 8245_2p -script e.tcl real 0m0.241s user 0m0.096s sys 0m0.043s
Which is quite an impressive improvement I must say!
It makes the boot a lot quicker, thanks for the good tip NYYRIKKI