At the heart of the memory selection is the programmable array logic ( PAL) chip IC36. It selects which screen RAM is in use (normal or shadow); it controls the sideways ROM select latch IC45; it selects the paged RAM. 5.4.1 ROM operation Any ROM socket on the PCB can either hold an 8K, 16K or 32K BYTE device. 8K or 16K IC's are paged into the memory map from &8000-&9FFF or from &8000-&BFFF respectively, a 32K device provides two 16K banks of memory paged into the memory map from &8000-&BFFF. The extra address line A14 (QA from IC45) required by 32K devices is available to each ROM slot when the appropriate molex link is altered.
The ROM socket IC numbers, their corresponding ROM select numbers and their corresponding link numbers are shown in figure 4. IC No ROM Nos Link No Notes IC35 2/3 S9 Molex link made W as standard for 8K/16K use IC44 4/5 Sll " " " IC57 6/7 S12 " " " IC62 8/9 S15 " " " IC68 10/11 S18 " " " IC71 0/1 or S19 PCB cuttable link made E as standard for 32K 14/15 (16K operating system and I/O and 16K BASIC) Figure 4 ROM socket IC numbers, ROM numbers, and device selection link numbers As can be seen from figure 4, ROM numbers range from 0 to 15. ROMs are prioritised, the highest ROM number language and filing system will be selected after a 'hard' reset. 15 has the highest priority, 0 the lowest.
So if two or more sideways ROMs are language ROMs, then the computer will start up in the language in the highest number ROM slot. Similarly for filing system ROMS. IC35, 44, 57, 62, and 68 we shall call "user ROMs". Each user ROM socket is functionally identical and can contain language or service ROMs. IC71 we shall call the "system ROM". The system ROM contains the operating system. The operating system is always in the memory map from &C000-&FFFF and must always be fitted in IC71. As standard, the computer ones with a 32K ROM for IC71. It contains the operating system and the BASIC language. For this reason link S19 is hard wired East in the 32K position.
The BASIC part of the system ROM occupies one of four sideways ROM numbers. As standard, any call made to ROM 14 or 15 selects BASIC, and any call to ROM 0 or 1 is ignored. Hence BASIC occupies the highest priority ROM slot and the computer will start up in BASIC. If molex link S13 is moved from South to North then any call made to ROM 0 or 1 will select BASIC, and any call to ROM 14 or 15 will be ignored. This allows the user to select an alternative language at power-on (the language entered at start-up will be the one with the highest socket number when more than one language ROM is fitted). 15