{"id":48,"date":"2013-07-24T17:08:32","date_gmt":"2013-07-24T17:08:32","guid":{"rendered":"https:\/\/blogs.scummvm.org\/t0by\/?p=48"},"modified":"2022-05-24T17:09:56","modified_gmt":"2022-05-24T17:09:56","slug":"introducing-the-debugger","status":"publish","type":"post","link":"https:\/\/blogs.scummvm.org\/t0by\/2013\/07\/24\/introducing-the-debugger\/","title":{"rendered":"Introducing the debugger"},"content":{"rendered":"

\"\"<\/a>I have been cooking up a little debugger for a while.
\nNow, writing a debugger for the Wintermute Engine is a bit weird, because things do work in a slightly peculiar fashion.
\nFor example, you can have no debugging symbols, because that would mean hacking the compiler, which is not part of the ScummVM package.
\nBut, even more interesting: you don\u2019t need any.<\/p>\n

Here\u2019s the deal: each script runs inside its own \u201cVM\u201d \u2013 let\u2019s call it that \u2013 with its iP, stack and execution unit.
\nThere is a scheduler that handles the various VMs, creates new ones (e.g. on event) and kills off those that have finished the execution of their scripts.
\nThe \u201cexecution unit\u201d is a method that gets called once per instruction, evaluates the instruction and advances the IP.<\/p>\n

Like this:<\/p>\n

bool ScScript::executeInstruction() {\r\n    \/\/ ... \r\n    ScValue *op1; \r\n    ScValue *op2;\r\n    uint32 inst = getDWORD();\r\n    switch (inst) {\r\n \r\n    case II_DEF_VAR:\r\n        \/\/ ... \r\n    case II_DEF_GLOB_VAR:\r\n        \/\/ ... \r\n    case II_DEF_CONST_VAR: {\r\n        \/\/ ...\r\n    case II_RET:\r\n        \/\/ ...\r\n        \/\/ ...\r\n    case II_ADD:\r\n        op2 = _stack->pop();\r\n        op1 = _stack->pop();\r\n \r\n        if (op1->isNULL() || op2->isNULL()) {\r\n            _operand->setNULL();\r\n        } else if (op1->getType() == VAL_STRING || op2->getType() == VAL_STRING) {\r\n            char *tempStr = new char [strlen(op1->getString()) + strlen(op2->getString()) + 1];\r\n            strcpy(tempStr, op1->getString());\r\n            strcat(tempStr, op2->getString());\r\n            _operand->setString(tempStr);\r\n            delete[] tempStr;\r\n        } else if (op1->getType() == VAL_INT && op2->getType() == VAL_INT) {\r\n            _operand->setInt(op1->getInt() + op2->getInt());\r\n        } else {\r\n            _operand->setFloat(op1->getFloat() + op2->getFloat());\r\n        }\r\n \r\n        _stack->push(_operand);\r\n \r\n        break;\r\n         \r\n    case II_SUB: {\r\n        \/\/ ...\r\n    }\r\n    case II_DBG_LINE: {\r\n        \/\/ ...\r\n    }\r\n    default:\r\n        _gameRef->LOG(0, \"Fatal: Invalid instruction %d ('%s', line %d, IP:0x%x)\\n\", inst, _filename, _currentLine, _iP - sizeof(uint32));\r\n        _state = SCRIPT_FINISHED;\r\n        ret = STATUS_FAILED;\r\n    } \r\n    return ret;\r\n}\r\n<\/pre>\n
So do we do?
\nWe put hooks in there, ideally.<\/p>\n
This way, each time an instruction is executed we can look around and see what\u2019s happening.
\nWe can implement watch, breakpoints and stuff.
\nOh, but then there\u2019s the problem of knowing where the hell we are first.
\nOkay \u2013 watching can easily implemented all the same, by keeping a watch list and keeping track of what has changed since last time, but breaking?
\nWe have no debug symbols, so how do we know WHERE we are?
\nInterestingly, the WME team has been so kind as to add a special instruction to the instruction set: II_DBG_LINE.
\nGuess what it does?
\nIt updates a counter, _line, in a way that it always contains the number of the current line.<\/p>\n
What we do then is to place our hooks in a way to monitor that number and act accordingly \u2013 e.g. we have a list of breakpoints, if the current line is in there, we break \u2013 that is, we open the debugger window like this:<\/p>\n
case II_DBG_LINE: {\r\n    int newLine = getDWORD();\r\n    if (newLine != _currentLine) {\r\n        _currentLine = newLine;\r\n    }\r\n    for (int j = 0; j < _engine->_breakpoints.size(); j++) {\r\n        if (_engine->_breakpoints[j]._line == _currentLine &&\r\n            !strcmp(_engine->_breakpoints[j]._filename.c_str(), _filename)) { \r\n                _engine->_breakpoints[j]._hits++;\r\n                _adapter->triggerBreakpoint(this); \r\n                \/\/ This one breaks execution and does housekeeping.\r\n                \/\/ We resume from here once user stops execution\r\n            }\r\n        }\r\n    }\r\n \r\n    if (1) { \r\n        if (_callStack->_sP <= _step) { \/\/ This is set elsewhere, suffice saying that I may or may not want to enter a function past a certain call depth _adapter->triggerStep(this);\r\n        }\r\n    }\r\n \r\n    if (1) { \r\n             for (int i = 0; i < _watchlist.size(); i++) {\r\n                \/\/ Etc\r\n            } \r\n        }\r\n    }\r\n    break;\r\n<\/pre>\n
Then, we wait for user input.
\nHow this is handled we\u2019ll see in the next post.<\/p>\n","protected":false},"excerpt":{"rendered":"
I have been cooking up a little debugger for a while. Now, writing a debugger for the Wintermute Engine is a bit weird, because things do work in a slightly peculiar fashion. For example, you can have no debugging symbols, because that would mean hacking the compiler, which is not part of the ScummVM package. […]<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[1],"tags":[],"class_list":["post-48","post","type-post","status-publish","format-standard","hentry","category-uncategorized"],"_links":{"self":[{"href":"https:\/\/blogs.scummvm.org\/t0by\/wp-json\/wp\/v2\/posts\/48","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/blogs.scummvm.org\/t0by\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/blogs.scummvm.org\/t0by\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/blogs.scummvm.org\/t0by\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/blogs.scummvm.org\/t0by\/wp-json\/wp\/v2\/comments?post=48"}],"version-history":[{"count":2,"href":"https:\/\/blogs.scummvm.org\/t0by\/wp-json\/wp\/v2\/posts\/48\/revisions"}],"predecessor-version":[{"id":51,"href":"https:\/\/blogs.scummvm.org\/t0by\/wp-json\/wp\/v2\/posts\/48\/revisions\/51"}],"wp:attachment":[{"href":"https:\/\/blogs.scummvm.org\/t0by\/wp-json\/wp\/v2\/media?parent=48"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/blogs.scummvm.org\/t0by\/wp-json\/wp\/v2\/categories?post=48"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/blogs.scummvm.org\/t0by\/wp-json\/wp\/v2\/tags?post=48"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}