A comparison...

RAM ADDRESSING / SIZES
> fixed 64 K RAM on board (instead of 128 K on board of PC-100 B)

> a maximum of 832 K RAM -with RAM extension board - (instead of 896 K on 'B')

ROM
> ROM 04.03.11 (for PC-100-A) in 3 chips (type 2764)
FA000-FBFFF (E89)
FC000-FDFFF (E90)
FE000-FFFFF (E91)

> PC-100B / PC100B+ has ROM 05.03 in 2 chips (type 27128) at memory addresses:
F4000-F7FFF
FC000-FFFFF

ROM DIFFERENCES (derived from my latest ROM disassembly)

(*) PC-100-B only. Not present in original PC-100 specification from March 1983.
0x1a and beyond are marked as 'reserved' there.
Code
$1f23 - (*) ROM_RomVersion       0x1a  /* DX,BP<-offset,seg of 8-byte buffer */
$1f66 - (*) ROM_ChangeVectorMap  0x1c  
$1fc2 - (*) ROM_RingBell         0x1e
$1fc8 - (*) ROM_GetSet7or8       0x20  /* AH<-function (1=get, 0=set);       */
                                       /* AL<-value (0=8, 1=7) (set only) :  */
                                       /*     value->AL (get only)           */
There is no documented way to emit bell sounds on the Rainbow 100-A (confirmed in Jeff Armstrong's blog). No hardware limitation of course: the beeper sits in the keyboard.

The 'A' model did not come with a hard disc, and there were no provisions in the old ROM to boot from HD directly.

LIMITS WITH OPTION BOARDS

GRAPHICS (when using uPD7220 Graphics option)
> limited graphics palette: 4 colors where with the B board you get 16.

80286 ACCELERATOR BOARD
> Suitable Solutions 286 accelerator board runs in either a 100B or a 100+, but not a 100A.

Note that i own neither a 100-A nor the fancier option boards. I am also more into software than hardware. Feel free to correct me. smile

Last edited by Bavarese; 02/19/14 09:05 PM.