Bug: The return pointer in TRAP is still saved using R7

[yezhiyi9670]

TRAP routines seem to modify R7 to the incremented PC, causing R7 to be changed unexpectedly.

.ORIG x3000

            LEA R0, STR1
            AND R7, R7, #0
            ADD R7, R7, #3
            PUTS            ; Call PUTS
            LD R0, CHR48
            ADD R0, R0, R7  ; Convert R7 to character
            OUT             ; Output R7 as digit

            HALT

STR1        .STRINGZ "Message\n"
CHR48       .FILL x30

.END

[yezhiyi9670]

In this version of LC-3 handbook, PC is saved using R7. However, according to the appendix of the latest textbook, the incremented PC is directly pushed onto the system stack. TRAP should do this instead:

TEMP = PSR;
if (PSR[15] == 1)
   Saved_USP = R6 and R6 = Saved_SSP
   PSR[15] = 0
Push TEMP.PC on the system stack
PC = mem[ZEXT(trapvect8)];