Skip to navigation

Elite on the BBC Micro

Loader: STARTUP (Master version)

Name: STARTUP [View in context] Type: Subroutine [Compare versions] Category: Loader Summary: Set the various vectors, interrupts and timers
.STARTUP SEI \ Disable interrupts LDA #%00111001 \ Set 6522 System VIA interrupt enable register IER STA VIA+&4E \ (SHEILA &4E) bits 0 and 3-5 (i.e. disable the Timer1, \ CB1, CB2 and CA2 interrupts from the System VIA) LDA #%01111111 \ Set 6522 User VIA interrupt enable register IER STA &FE6E \ (SHEILA &6E) bits 0-7 (i.e. disable all hardware \ interrupts from the User VIA) LDA IRQ1V \ Store the current IRQ1V vector in VEC, so VEC(1 0) now STA VEC \ contains the original address of the IRQ1 handler LDA IRQ1V+1 STA VEC+1 LDA #LO(IRQ1) \ Set the IRQ1V vector to IRQ1, so IRQ1 is now the STA IRQ1V \ interrupt handler LDA #HI(IRQ1) STA IRQ1V+1 LDA VSCAN \ Set 6522 System VIA T1C-L timer 1 high-order counter STA VIA+&45 \ (SHEILA &45) to the contents of VSCAN (57) to start \ the T1 counter counting down from 14592 at a rate of \ 1 MHz CLI \ Enable interrupts again RTS \ Return from the subroutine