The following program executes, generating an interrupt for DECI: 0000 040005 BR

Question

The following program executes, generating an interrupt for DECI:

0000 040005 BR main ;Branch around data
0003 0000 num: .BLOCK 2 ;Global variable
;
0005 310003 main: DECI num,d ;Input decimal value
0008 390003 DECO num,d ;Output decimal value
000B 50000A CHARO ' ',i
000E 410012 STRO msg,d ;Output message
0011 00 STOP
0012 546861 msg: .ASCII "That's all. "
742773
20616C
6C2E0A
00
001F .END

The entry to and exit from the trap handler, state the values in the following registers as four hexadecimal digits:


(c)X after SUBX at FCBA

Explanation / Answer

You can simply run this along with the second part

;******* Trap handl er
oldIR:   .EQUATE 9          ;Stack address of IR on trap
;
FC9B C80000 trap:    LDX     0,i        ;Cl ear X for a byte compare
FC9E DB0009          LDBYTEX ol dIR,s    ;X := trapped IR
FCA1 B80028          CPX     0x0028,i   ;If X >= first nonunary trap opcode
FCA4 0EFCB7          BRGE    nonUnary   ;trap opcode is nonunary
;
FCA7 980003 unary:   ANDX    0x0003,i   ;Mask out al l but rightmost two bits
FCAA 1D              ASLX               ;An address is two bytes
FCAB 17FCAF          CALL    unaryJT,x ;Cal l unary trap routine
FCAE 01              RETTR              ;Return from trap
;
FCAF FDB6   unaryJT: .ADDRSS opcode24   ;Address of NOP0 subroutine
FCB1 FDB7            .ADDRSS opcode25   ;Address of NOP1 subroutine
FCB3 FDB8            .ADDRSS opcode26   ;Address of NOP2 subroutine
FCB5 FDB9            .ADDRSS opcode27   ;Address of NOP3 subroutine
;
FCB7 1F     nonUnary:ASRX               ;Trap opcode is nonunary
FCB8 1F              ASRX               ;Discard addressing mode bits
FCB9 1F              ASRX
FCBA 880005          SUBX    5,i        ;Adjust so that NOP opcode = 0
FCBD 1D              ASLX               ;An address is two bytes
FCBE 17FCC2          CALL    nonUnJT,x ;Cal l nonunary trap routine
FCC1 01     return: RETTR              ;Return from trap
;
FCC2 FDBA   nonUnJT: .ADDRSS opcode28   ;Address of NOP subroutine
FCC4 FDC4            .ADDRSS opcode30   ;Address of DECI subroutine
FCC6 FF3B            .ADDRSS opcode38   ;Address of DECO subroutine
FCC8 FFC6            .ADDRSS opcode40   ;Address of STRO subroutine

And step through the code until you get to the part at FCBA which gives you 0001 for your index register

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