Editing Diagnostic Serial Console (section)

=== Assembly Language Implementation Details ===

The diagnostic interface implements memory testing algorithms through sophisticated 68000 assembly routines that directly manipulate memory controller hardware. Based on ROM disassembly projects and vintage computing research, the implementation follows structured patterns typical of professional diagnostic code from Apple's era.

'''Memory Test Loop Structure (typical implementation):'''
<pre>
MEMTEST_LOOP:
    MOVE.L   #$00000000,A0    ; Initialize base address pointer  
    MOVE.L   #$003FFFFC,A1    ; Set end address (4MB boundary)
    MOVEQ    #$55,D0          ; Load test pattern (0x55555555)
    LSL.L    #8,D0            
    OR.L     #$55,D0
    LSL.L    #8,D0
    OR.L     #$55,D0
    LSL.L    #8,D0
    OR.L     #$55,D0          ; D0 now contains $55555555

WRITE_PHASE:
    MOVE.L   D0,(A0)+         ; Write pattern to memory, increment pointer
    CMPA.L   A1,A0            ; Compare current with end address
    BLS.S    WRITE_PHASE      ; Continue if not at end

    MOVE.L   #$00000000,A0    ; Reset to base address

READ_PHASE:
    MOVE.L   (A0)+,D1         ; Read memory value, increment pointer
    CMP.L    D0,D1            ; Compare with expected pattern
    BNE.S    MEMORY_ERROR     ; Branch if mismatch found
    CMPA.L   A1,A0            ; Check if at end
    BLS.S    READ_PHASE       ; Continue if not at end
    RTS                       ; Return if test passed

MEMORY_ERROR:
    SUBA.L   #4,A0            ; Back up to failing address
    MOVE.L   A0,D2            ; Store error address
    MOVE.L   D1,D3            ; Store read value
    EOR.L    D0,D3            ; XOR with expected (shows differing bits)
    RTS                       ; Return with error information
</pre>

'''Walking Bit Pattern Test Implementation:'''
<pre>
WALKING_ONES_TEST:
    MOVE.L   #$00000000,A0    ; Base memory address
    MOVEQ    #31,D7           ; Loop counter (32 bit positions)
    MOVEQ    #1,D0            ; Initial walking bit pattern

WALK_LOOP:
    MOVE.L   D0,(A0)          ; Write pattern to memory
    MOVE.L   (A0),D1          ; Read back immediately
    CMP.L    D0,D1            ; Compare written vs read
    BNE.S    BIT_ERROR        ; Branch if mismatch
    LSL.L    #1,D0            ; Shift bit left for next position
    DBRA     D7,WALK_LOOP     ; Decrement and branch if not done
    RTS                       ; Return success

BIT_ERROR:
    MOVE.L   D0,D2            ; Store failing pattern
    MOVE.L   D1,D3            ; Store read value
    RTS                       ; Return with error data
</pre>

'''Error Code Generation Routine:'''
<pre>
GENERATE_ERROR_CODE:
    MOVE.L   D2,D0            ; Error address in D2
    LSR.L    #8,D0            ; Shift to get upper bits
    LSR.L    #8,D0
    ANDI.L   #$0000FFFF,D0    ; Mask to 16 bits
    MOVE.W   D0,ERROR_HI      ; Store high word of error code
    
    MOVE.L   D3,D0            ; Failing data pattern in D3
    ANDI.L   #$0000FFFF,D0    ; Mask to 16 bits  
    MOVE.W   D0,ERROR_LO      ; Store low word of error code
    
    OR.L     #$FFFF0000,D0    ; Set error flag bits
    MOVE.L   D0,DIAGNOSTIC_RESULT ; Store complete diagnostic code
    RTS
</pre>

Address decode testing routines utilize the 68030's addressing capabilities to systematically validate memory controller functionality across the complete address space. The implementation includes specialized routines for bank switching validation, address line testing through walking bit patterns, and data bus integrity verification through structured pattern testing. Error reporting utilizes hexadecimal encoding for compact representation of complex diagnostic states.

'''March Algorithm Implementation (based on community diagnostic ROM projects):'''
<pre>
MARCH_C_MINUS_TEST:
    ; Step 1: Initialize all memory with 0s (ascending)
    MOVE.L   #MEMORY_BASE,A0
    MOVE.L   #MEMORY_END,A1
    MOVEQ    #0,D0
INIT_LOOP:
    MOVE.L   D0,(A0)+
    CMPA.L   A1,A0
    BLS.S    INIT_LOOP

    ; Step 2: Read 0, Write 1 (ascending)
    MOVE.L   #MEMORY_BASE,A0
    MOVEQ    #-1,D1           ; $FFFFFFFF pattern
STEP2_LOOP:
    MOVE.L   (A0),D2          ; Read current value
    TST.L    D2               ; Should be 0
    BNE.S    MARCH_ERROR      ; Error if not 0
    MOVE.L   D1,(A0)+         ; Write 1s pattern
    CMPA.L   A1,A0
    BLS.S    STEP2_LOOP

    ; Step 3: Read 1, Write 0 (ascending)  
    MOVE.L   #MEMORY_BASE,A0
STEP3_LOOP:
    MOVE.L   (A0),D2          ; Read current value
    CMP.L    D1,D2            ; Should be all 1s
    BNE.S    MARCH_ERROR      ; Error if not 1s
    MOVE.L   D0,(A0)+         ; Write 0s pattern
    CMPA.L   A1,A0
    BLS.S    STEP3_LOOP
    RTS                       ; Test passed

MARCH_ERROR:
    SUBA.L   #4,A0            ; Back up to error address
    MOVE.L   A0,D4            ; Store error location
    MOVE.L   D2,D5            ; Store unexpected value
    RTS                       ; Return with error info
</pre>