Editing Diagnostic Serial Console (section)

=== ROM-Based Diagnostic Code Architecture ===

The SE/30's diagnostic capabilities are implemented through sophisticated 68000 assembly language routines embedded within the system ROM at address 0x40800000. These diagnostic routines operate independently of Mac OS, providing direct hardware access and component-level testing through carefully structured assembly code that bypasses normal operating system abstractions. 

'''Boot Sequence and Diagnostic Entry Points (from ROM disassembly projects):'''
<pre>
; SE/30 ROM Boot Vector Table (typical structure)
    ORG    $40800000         ; ROM base address
    DC.L   $40810000         ; Initial stack pointer
    DC.L   BOOT_START        ; Initial program counter
    DC.L   BUS_ERROR_HANDLER ; Bus error vector
    DC.L   ADDR_ERROR_HANDLER; Address error vector
    ; ... additional vectors

BOOT_START:
    ; Disable interrupts and initialize CPU state
    ORI.W  #$0700,SR         ; Disable all interrupts
    MOVE.L #$40810000,A7     ; Set stack pointer
    
    ; Test ROM checksum first
    BSR    ROM_CHECKSUM_TEST
    TST.L  D0
    BNE    ROM_FAILURE
    
    ; Initialize diagnostic state
    MOVE.L #DIAGNOSTIC_VARS,A6  ; Base pointer for diagnostic variables
    CLR.L  ERROR_CODE(A6)       ; Clear error accumulator
    
    ; Begin hardware initialization sequence
    BSR    INIT_VIA_CHIPS
    BSR    INIT_SCC_SERIAL
    BSR    INIT_MEMORY_CONTROLLER
    
    ; Start memory testing if in diagnostic mode
    BTST   #DIAG_MODE_BIT,SYSTEM_FLAGS
    BNE    DIAGNOSTIC_MAIN
    
    ; Normal boot continues here...
    BRA    NORMAL_BOOT_SEQUENCE
</pre>

'''Serial Diagnostic Interface Handler:'''
<pre>
DIAGNOSTIC_SERIAL_HANDLER:
    ; Check for incoming serial command
    MOVE.B SCC_STATUS,D0      ; Read SCC status register
    BTST   #RX_READY_BIT,D0   ; Check receive ready
    BEQ.S  CHECK_OUTPUT       ; No input, check output
    
    ; Read command character
    MOVE.B SCC_DATA,D0        ; Get character from SCC
    CMPI.B #'*',D0            ; Check for command prefix
    BNE.S  STORE_DATA         ; Not command, store as data
    
    ; Process diagnostic command
    BSR    READ_COMMAND_STRING
    BSR    PARSE_DIAGNOSTIC_CMD
    BSR    EXECUTE_DIAGNOSTIC
    BRA.S  SEND_RESPONSE

STORE_DATA:
    ; Store data for memory operations
    MOVE.L CURRENT_DATA_PTR(A6),A0
    MOVE.B D0,(A0)+
    MOVE.L A0,CURRENT_DATA_PTR(A6)
    RTS

CHECK_OUTPUT:
    ; Check if we need to send data
    TST.B  OUTPUT_PENDING(A6)
    BEQ.S  HANDLER_EXIT
    BSR    SEND_DIAGNOSTIC_DATA
    
HANDLER_EXIT:
    RTS
</pre>

'''Diagnostic Command Parser:'''
<pre>
PARSE_DIAGNOSTIC_CMD:
    ; Input: Command string in buffer pointed to by A0
    ; Parse command type (*T, *L, *D, *M, etc.)
    MOVE.B 1(A0),D0           ; Get command character after '*'
    
    CMPI.B #'T',D0            ; Test command?
    BEQ    HANDLE_TEST_CMD
    CMPI.B #'L',D0            ; Location set command?
    BEQ    HANDLE_LOCATION_CMD  
    CMPI.B #'D',D0            ; Data deposit command?
    BEQ    HANDLE_DEPOSIT_CMD
    CMPI.B #'M',D0            ; Memory read command?
    BEQ    HANDLE_MEMORY_READ
    CMPI.B #'G',D0            ; Go/execute command?
    BEQ    HANDLE_EXECUTE_CMD
    
    ; Unknown command
    MOVE.L #CMD_ERROR,RESPONSE_PTR(A6)
    RTS

HANDLE_TEST_CMD:
    ; Parse test number (*T0001, *T0002, etc.)
    ADDQ.L #2,A0              ; Skip "*T"
    BSR    PARSE_HEX_NUMBER    ; Parse test number into D0
    MOVE.L D0,TEST_NUMBER(A6)
    
    ; Dispatch to appropriate test
    CMPI.L #$0001,D0
    BEQ    DATA_BUS_TEST
    CMPI.L #$0002,D0  
    BEQ    MOD3_RAM_TEST
    CMPI.L #$0003,D0
    BEQ    ADDRESS_LINE_TEST
    ; ... additional test dispatches
    
    RTS
</pre>

The diagnostic ROM implementation utilizes direct register manipulation and bus control to achieve precise hardware validation impossible through higher-level software interfaces. Memory allocation operates through direct address specification rather than dynamic allocation, with the system using register A6 for return address management during early boot phases when no stack is available. Critical timing relationships are maintained through carefully calculated instruction cycles and NOPs to ensure proper hardware setup and response timing.