add rp6502 target

libsrc/rp6502/irq.s

@@ -0,0 +1,51 @@
+; Copyright (c) 2023 Rumbledethumps
+;
+; SPDX-License-Identifier: Zlib
+; SPDX-License-Identifier: BSD-3-Clause
+; SPDX-License-Identifier: Unlicense
+
+.export initirq, doneirq
+.import callirq, _exit
+
+.include "rp6502.inc"
+
+.segment "ONCE"
+
+initirq:
+    lda #<handler
+    ldx #>handler
+    sei
+    sta $FFFE
+    stx $FFFF
+    cli
+    rts
+
+.code
+
+doneirq:
+    sei
+    rts
+
+.segment "LOWCODE"
+
+handler:
+    cld
+    phx
+    tsx
+    pha
+    inx
+    inx
+    lda $100,X
+    and #$10
+    bne break
+    phy
+    jsr callirq
+    ply
+    pla
+    plx
+    rti
+
+break:
+    lda #$FF
+    sta RIA_A
+    jmp _exit