Bochs Developers Guide
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# I/O Interface to Bochs Debugger

This device was added by Dave Poirier (eks@void-core.2y.net).

Compiling Bochs with iodebug support

    ./configure --enable-iodebug
    make

Other optional fields may be added to the ./configure line, see Bochs documentation for all the information.

    Using the I/O Interface to the debugger

    port range:  0x8A00 - 0x8A01

    Port 0x8A00 servers as command register. You can use it to enable the i/o interface,
    change which data register is active, etc.

    Port 0x8A01 is used as data register for the memory monitoring.

## Commands supported by port 0x8A00

    0x8A00

     Used to enable the device. Any I/O to the debug module before this command is sent
     is sent will simply be ignored.

    0x8A01

     Selects register 0: Memory monitoring range start address (inclusive)

    0x8A02

     Selects register 1: Memory monitoring range end address (exclusive)

    0x8A80

     Enable address range memory monitoring as indicated by register 0 and 1 and
     clears both registers

    0x8AE0 - Return to Debugger Prompt

     If the debugger is enabled (via --enable-debugger), sending 0x8AE0 to port 0x8A00
     after the device has been enabled will return the Bochs to the debugger prompt.
     Basically the same as doing CTRL+C.

    0x8AE2 - Instruction Trace Disable

     If the debugger is enabled (via --enable-debugger), sending 0x8AE2 to port 0x8A00
     after the device has been enabled will disable instruction tracing

    0x8AE3 - Instruction Trace Enable

     If the debugger is enabled (via --enable-debugger), sending 0x8AE3 to port 0x8A00
     after the device has been enabled will enable instruction tracing

    0x8AE4 - Register Trace Disable

     If the debugger is enabled (via --enable-debugger), sending 0x8AE4 to port 0x8A00
     after the device has been enabled will disable register tracing.

    0x8AE5 - Register Trace Enable

     If the debugger is enabled (via --enable-debugger), sending 0x8AE5 to port 0x8A00
     after the device has been enabled will enable register tracing. This currently
     output the value of all the registers for each instruction traced.
     Note: instruction tracing must be enabled to view the register tracing

    0x8AFF

     Disable the I/O interface to the debugger and the memory monitoring functions.

| all accesses must be done using word

| reading this register will return 0x8A00 if currently activated, otherwise 0

## Access to port 0x8A01 (write-only)

All accesses to this port must be done using words. Writing to this port will shift to the left by 16 the current value of the register and add the provided value to it.

    Sample:

    reg0 = 0x01234567

    out port: 0x8A01 data: 0xABCD

    reg0 = 0x4567ABCD

## Sample

Enable memory monitoring on first page of text screen (0xb8000-0xb8fa0): add in bochrc file: optromimage1: file="asmio.rom", address=0xd0000

    /*
     *      Make asmio ROM file:
     *      gcc -c asmio.S
     *      objcopy -O binary asmio.o asmio.rom
     */
            .text
            .global start
            .code16

    /* ROM Header */
            .byte 0x55
            .byte 0xAA
            .byte 1                 /* 512 bytes long */

    start:
    /* Monitor memory access on first page of text screen */
            mov     $0x8A00,%dx     /* Enable iodebug (0x8A00->0x8A00) */
            mov     %dx,%ax
            out     %ax,%dx
            mov     $0x8A01,%ax     /* Select register 0 start addr (0x8A01->0x8A00) */
            out     %ax,%dx
            mov     $0x8A01,%dx     /* Write start addr 0xB8000 (high word first) */
            mov     $0xB,%ax
            out     %ax,%dx
            mov     $0x8000,%ax     /* Write start addr (low word) */
            out     %ax,%dx

            mov     $0x8A02,%ax     /* Select register 1 end addr (0x8A02->0x8A00) */
            mov     $0x8A00,%dx
            out     %ax,%dx
            mov     $0x8A01,%dx     /* Write end addr 0xB8FA0 (high word first) */
            mov     $0xB,%ax
            out     %ax,%dx
            mov     $0x8FA0,%ax     /* Write end addr (low word) */
            out     %ax,%dx

            mov     $0x8A00,%dx     /* Enable addr range memory monitoring (0x8A80->0x8A00) */
            mov     $0x8A80,%ax
            out     %ax,%dx

            mov     $0x8A00,%dx     /* Return to Bochs Debugger Prompt (0x8AE0->0x8A00) */
            mov     $0x8AE0,%ax
            out     %ax,%dx
            lret

            .byte 0x6b              /* Checksum (code dependent!, update it as needed) */
            .align 512              /* NOP follow */

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