CNP-1 - BIOS & Interrupt Vectors

BIOS Calls

BIOS calls are indexed into a “jump table” allowing higher level code to call them without knowing the underlying addresses. This allows the ROM routine implementations to move around without needing to re-assemble/compile code that accesses them. These are only used for code that is assembled outside of the rom_image folder. This is very much in flux, as I haven’t used it for anything but some testing yet.

BIOSJMPTABLE            = $FF00
 write_bufferln         = $00
 write_buffer           = $02
 write_char             = $04
 read_char              = $06
 get_line               = $08
 char_avail             = $0A
 read_char_nonblocking  = $0C

Interrupt Vectors

Reset: Points to the BIOS initialization routine
IRQ: Right now is a dummy routine
NMI: I use the NMI on these systems as a last ditch effort troubleshooting measure. Triggering an NMI will cause the system to dump the current processor registers and return from the interrupt. This is remniscent of the message that is printed when an Apple IIe “crashes” to its monitor. Disclaimer: This wasn’t my idea, it likely came from 6502.org or Garth Wilson’s page.

.export nmi_handler
.proc nmi_handler
    cld
				
    pha                           ; Preserve registers
    phx
    phy
    tsx                           ; Preserve stack pointer

    putsln ""
    putsln "NMI received"
								
    puts "SP:"
    txa                           ; Print stack pointer
    adc #5
    jsr print_byte_as_hex_chars
				
    puts " Y:"
    inx
    lda $0100,x
    jsr print_byte_as_hex_chars
				
    puts " X:"
    inx
    lda $0100,x
    jsr print_byte_as_hex_chars
				
    puts " A:"
    inx
    lda $0100,x
    jsr print_byte_as_hex_chars
				
    inx
    inx
    puts " PC:"
    inx
    lda $0100,x
    jsr print_byte_as_hex_chars
    dex
    lda $0100,x
    jsr print_byte_as_hex_chars
				
    putsln ""
				
    ply                           ; Restore registers
    plx
    pla
				
    rti
.endproc