amiga-bootcamp/07_dos/packet_system.md

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Packet System — DosPacket, ACTION_* Codes

Overview

AmigaDOS filesystem handlers communicate via DosPackets — messages sent to the handler's MsgPort. Every file operation (Open, Read, Lock, etc.) is internally translated into an ACTION_* packet. Understanding packets is essential for writing custom handlers or intercepting filesystem calls.

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struct DosPacket

/* dos/dosextens.h — NDK39 */
struct DosPacket {
    struct Message *dp_Link;  /* exec message (backlink) */
    struct MsgPort *dp_Port;  /* reply port */
    LONG   dp_Type;           /* ACTION_* code */
    LONG   dp_Res1;           /* primary result */
    LONG   dp_Res2;           /* secondary result (error code) */
    LONG   dp_Arg1;           /* argument 1 — type depends on dp_Type */
    LONG   dp_Arg2;
    LONG   dp_Arg3;
    LONG   dp_Arg4;
    LONG   dp_Arg5;
    LONG   dp_Arg6;
    LONG   dp_Arg7;
};

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Common ACTION_* Codes

Code	Dec	Action	Args
ACTION_FINDINPUT	1005	Open for reading	Arg1=FileHandle, Arg2=Lock, Arg3=name(BSTR)
ACTION_FINDOUTPUT	1006	Open for writing (create)	same
ACTION_FINDUPDATE	1004	Open for r/w	same
ACTION_READ	82	Read bytes	Arg1=FH_Arg1, Arg2=buf, Arg3=len
ACTION_WRITE	87	Write bytes	Arg1=FH_Arg1, Arg2=buf, Arg3=len
ACTION_SEEK	1008	Seek	Arg1=FH_Arg1, Arg2=pos, Arg3=mode
ACTION_END	1007	Close file	Arg1=FH_Arg1
ACTION_LOCATE_OBJECT	8	Lock (obtain)	Arg1=dirLock, Arg2=name(BSTR), Arg3=mode
ACTION_FREE_LOCK	15	UnLock	Arg1=lock
ACTION_EXAMINE_OBJECT	23	Examine (stat)	Arg1=lock, Arg2=FIB(BPTR)
ACTION_EXAMINE_NEXT	24	ExNext	Arg1=lock, Arg2=FIB(BPTR)
ACTION_PARENT	29	ParentDir	Arg1=lock
ACTION_DELETE_OBJECT	16	Delete	Arg1=lock, Arg2=name(BSTR)
ACTION_RENAME_OBJECT	17	Rename	Arg1=fromLock, Arg2=fromName, Arg3=toLock, Arg4=toName
ACTION_CREATE_DIR	22	CreateDir	Arg1=lock, Arg2=name(BSTR)
ACTION_SET_PROTECT	21	SetProtection	Arg1=0, Arg2=lock, Arg3=name(BSTR), Arg4=bits
ACTION_DISK_INFO	25	Info	Arg1=InfoData(BPTR)
ACTION_IS_FILESYSTEM	1027	Query	(none) → Res1=DOSTRUE if filesystem

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Sending a Packet Manually

struct MsgPort *handler = ((struct FileLock *)BADDR(lock))->fl_Task;
struct StandardPacket sp;
sp.sp_Msg.mn_Node.ln_Name = (char *)&sp.sp_Pkt;
sp.sp_Pkt.dp_Link = &sp.sp_Msg;
sp.sp_Pkt.dp_Port = CreateMsgPort();
sp.sp_Pkt.dp_Type = ACTION_DISK_INFO;
sp.sp_Pkt.dp_Arg1 = MKBADDR(infodata);
PutMsg(handler, &sp.sp_Msg);
WaitPort(sp.sp_Pkt.dp_Port);
GetMsg(sp.sp_Pkt.dp_Port);
/* sp.sp_Pkt.dp_Res1 = result */
DeleteMsgPort(sp.sp_Pkt.dp_Port);

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BSTR — BCPL Strings

Handler packets use BSTR for filenames: a BPTR to a length-prefixed string:

[len_byte][char_0][char_1]...[char_n]

• len_byte = string length (max 255)
• No null terminator
• Convert: UBYTE *bstr = (UBYTE *)BADDR(bstr_bptr); int len = bstr[0]; char *name = &bstr[1];

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References

• NDK39: dos/dosextens.h, dos/dos.h
• ADCD 2.1: DoPkt, packet system
• Amiga ROM Kernel Reference Manual: Devices — filesystem handler chapter