[syslinux] "isolinux.bin missing or corrupt" when booting USB flash drive in old PC

[Ady Ady]

>From Martin's isolinux.bin.20170324, I (kind of) understand the first 3 
bytes:
33 ED 90 

Then, the next "90"s are probably originiated from the "nop"s in:
 
http://repo.or.cz/syslinux.git/blobdiff/c210e571f61eeb898fadbb6f47d4a2c8
2e5bfbfd..2c3a24e5f4b807ec31595227afa59a818c060ca9:/mbr/isohdpfx.S 

So, why the original isohybrid image from Debian and many other 
isohybrid images have mostly (but not all) "00s" in place of the "90s"? 
(Considering that these images are generated mostly by xorriso, perhaps 
Thomas might be able to answer?)

To ilustrate what I mean (and what I'm trying to understand), I am 
presenting here the first part of Martin's isolinux.bin20170324 and the 
respective part from the original isohyrbid image from Debian (note: 
the strange format I am presenting them is intentional):

Martin's:

33 ED 90 
90 90 90 
90 90 
90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 

Original Debian's bin:
45 52 08 
00 00 00 
90 90 
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 

TIA,
Ady.