44 lines
1.0 KiB
ArmAsm
44 lines
1.0 KiB
ArmAsm
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vector_real:
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// Use MEMUSS as temporary storage location - checked on real C64 that this is the
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// address originally used; after calling VECTOR and checking zero page
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// area afterwards, the address could be found there
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// According to 'Compute's Mapping the Commodore 64' page 237,
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// the CBM implementation does not disable IRQs - yet, the
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// 'C64 Programmers Reference Guide' does not contain such
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// warning and does not mention any preparations needed, so...
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// better safe than sorry, disable the IRQ for the duration
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// of vector manipulation
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php
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cli
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// Prepare the user data pointer - strange order to reduce risk
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// of potential similarity to the original routine
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sty MEMUSS + 1
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stx MEMUSS + 0
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// Select routine variant - store or restore vectors
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ldy #$1F
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bcc vector_restore
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lda CINV, y
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sta (MEMUSS), y
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bcs vector_end_loop // branch always
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vector_restore:
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lda (MEMUSS), y
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sta CINV, y
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// FALLTROUGH
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vector_end_loop:
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dey
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bpl vector_restore
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// Restore status register (IRQ) before return
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plp
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rts
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