Sinclair ZX81, Assembly Instructions Manual, Page: 15 / 43

INCREMENTAL SERIES PLUG & SOCKET SWITCH SETTINGS APPENDIX B
General ITotes:
ITote the positions of the plug—and-socket switches (see Appendix A) and the numbering of
the possible positions of the 5 connector plugs. Five distinct ranges of positions
correspond to different selection functions described in detail below. There should normall
be one plug in each of these ranges.
-• i: L.I- _ i- -.-'.-
v
' •
MBSA (MemoryBoard §tart Address 1 0 (Start address of ZX81 8K ROM)
select;
2 RK
_
fUt f r £ Q locatlon above 8K R C M
j
Selects the position in the 64K address ^ ,
Alr
/„ , . . . _ . . „ ' < .
. space oT the
r
ZXBl microprocessor of the ^ ^ <
fcrmal
position of start of W _
mera0
^°
n
TT
he
™\>
oard+
; . -5
:I
3?r (1st Tree'location Shove~15K pack)
Normally set to ,27.30 that memory
f
,„„.
runs from address ^upwards.
? 4
g
K ( l g t f r e e l o c a t l o n above 32K
pack)
8 56K
9 Nowhere! (re memory board disabled)
10 MBSA ( Set if no memory on memory board)
11 MBSA+2K (Set for 2K (re 1 chip) on m boar<
ZES (ZX81 (IK) RAM select) , 12 MB3A+4K (2 chips on memory board)
13 MBSA-r6K (3 chips...
14 MBSA
T
8K (4 chijs...
Selects distance above MBSA of the 15 MBSA+10K (5 chips...
IK RAM on the ZX81 board thus allowing 16 MBSA+12K (6 chips...
it to be placed diately above 17 MESA+14K (? chips...
the memory on the memory board without 18 Ecohere! (le 2X81 RAM disabled)
any gaps and used normally.
19 Unused
20 Unused
BSS ((Memory) Board Block Size daloct) 21 Must be set if more than 8K on memory bn-r
22 Set if 8K or less on memory board.
If you wish the memory board to 23 Set if more than 8K on memory board.
occupy only 8K of address space must 24 Set if 8K or less on memory board.
set positions 22 and 24. Normal setting
is 21 and 23(f or 16K block size) .
TRS (Total RAM Select) 25 Set if total RAM not more than 16K.
_ , , -„. ,, „, „. ... 26 Set if total RAM is more than 16K.
Enables or Disables Adress line 15
to MBSA decoder,
27 Unused
Examples; Switch settings
Sa«4ware_Configuration MSA_^_52£__™5
1. ZX81 with original 1KRAM still on ZX81 board with Incremental
containing one 2K RAM chip. No external memory. 3 11 2^>23 25
2. ZX81 with original IK RAM still on ZX81 board with .r.cremental
containing full 16K of RAM. No external memory. 3 18 21,23 25
3. ZX81 with 16K add-on pack and original IK RAM still 5.1 ZX81 board {£.
with Incremental containing 2K of RAM. 5 2^ 21,23 26
4. ZX81 with . ' IK RAM still on ZX81 board and 48l external
add-on memory with Incremental containing 8K of memory to give
the maximum possible directly addressed RAM (56K = 6*. C - 8K ROM) 2 18 22,24 26