Electron ULA

Hardware initial page

&FE05

This register combines ROM selection and interupt acknowledge.

       b7 b6 b5 b4 b3 b2 b1 b0
 nmi   -+  |  |  |  |  |  |  |
 high  ----+  |  |  |  |  |  |
 rtc   -------+  |  |  |  |  |
 frame ----------+  |  |  |  |
 pe    -------------+  |  |  |
 p2    ----------------+  |  |
 p1    -------------------+  |
 p0    ----------------------+

• Writing %0000rrrr writes to the ROMSEL register
• Writing %nnnnxxxx where any n<>0 acknowledges an interupt and makes no change to the ROMSEL register

The select a paged ROM ROMSEL normally has to be written twice. First store &0C then store the ROM number:

 .SelectROM
 \ X=ROM number
 LDA #12:JSR SelectROM2:TXA
 .SelectROM2
 STA &F4:STA &FE05:RTS

Machine-specific code can be avoided by using OSRDRM.