{"id":15173,"date":"2010-01-26T07:00:00","date_gmt":"2010-01-26T07:00:00","guid":{"rendered":"https:\/\/blogs.msdn.microsoft.com\/oldnewthing\/2010\/01\/26\/microspeak-zap\/"},"modified":"2010-01-26T07:00:00","modified_gmt":"2010-01-26T07:00:00","slug":"microspeak-zap","status":"publish","type":"post","link":"https:\/\/devblogs.microsoft.com\/oldnewthing\/20100126-00\/?p=15173","title":{"rendered":"Microspeak: Zap"},"content":{"rendered":"

\nYou may hear an old-timer developer use the verb\nzap<\/i>.\n<\/p>\n

\nThat proposed fix will work.\nUntil everybody gets the fix,\nthey can just zap the assert.\n<\/p><\/blockquote>\n

\nThe verb to zap<\/i> means\nto replace a breakpoint instruction\nwith an appropriate number of NOP instructions<\/i>\n(effectively ignoring it).\n<\/p>\n

\nThe name comes from the old Windows 2.x kernel debugger.\n(Actually, it may be even older, but that’s as far back as I\nwas able to trace it.)\nThe Z<\/code> (zap<\/i>) command replaces the current\ninstruction with a NOP if it is an int 3<\/code>\n(the x86 single-byte breakpoint instruction),\nor replaced the previous instruction with NOPs if it is an\nint 1<\/code>\n(the x86 two-byte breakpoint instruction).\n<\/p>\n

\nThis operation was quite common back in the days when\nlots of code was written in assembly language.\nA technique used by some teams was to insert\na hard-coded breakpoint (called a TRAP<\/code>)\ninto every code path of a function.\nHere’s an example (with comments and other identifying characteristics\nremoved and new ones made up):\n<\/p>\n

\nxyz8:   mov     bl,[eax].xyz_State\n        cmp     bl,XYZSTATE_IGNORE\n        TRAPe\n        je      short xyz10     ; ignore this one\n        or      bl,bl\n        TRAPe\n        je      short xyz11     ; end of table\n        mov     bh,[eax].xyz_Flags\n        test    bh,XYZFLAGS_HIDDEN\n        TRAPz\n        jz      short xyz10     ; skip - item is hidden\n        test    bh,XYZFLAGS_MAGIC\n        TRAPe\n        je      short gvl10     ; skip - not the magic item\n        TRAP\n        bts     [esi].alt_flags,ALTFLAGS_SEENMAGIC\n        TRAPc\n        jc      short xyz10     ; weird - we shouldn't have two magic items\n<\/pre>\n
\nThere were a variety of TRAP<\/code> macros.\nHere we see the one plain vanilla TRAP<\/code> and a bunch\nof fancy traps which trigger only when certain conditions are met.\nFor example, TRAPc<\/code> traps if the carry is set.\nHere’s its definition:\n<\/p>\n
\nTRAPc   MACRO\n        local   l\n        jnc     short l\n        int     3\nl:\n        ENDM\n<\/pre>\n
\nHardly\n\nrocket science<\/a>.\n<\/p>\n
\nWhen you became the person to trigger a particular code path for\nthe first time,\nyou would trigger the trap, and you either stepped through\nthe code yourself or (if you weren’t familiar with the code)\ncontacted the author of the code to verify that the code\nsuccessfully handled this “never seen before” case.\nWhen sufficiently satisfied that a code path operated as expected,\nthe developer removed the corresponding TRAP<\/code>\nfrom the source code.\n<\/p>\n
\nOf course, most TRAP<\/code>s are removed before the code\ngets checked in, but the ones related to error handling or\nrecovering from data corruption\ntend to remain\n(such as here, where we inserted a TRAP<\/code> when we\nencounter two magic items, which is theoretically impossible).\n<\/p>\n
\nWhen you trigger one trap,\nyou usually trigger it a lot,\nand you usually trigger a lot of related traps as well.\nThe Z<\/code> command was quite handy at neutering each\none after you checked that everything was working.\nYou zapped the trap.\n<\/p>\n
\nThat’s why old-timers refer to patching out a hard-coded\nbreakpoint as zapping,\neven though the zap<\/i> command hasn’t existed for over a decade.\n<\/p>\n
\nUpdate<\/b>:\nAs far as I can tell, the earlier uses of the word zap<\/i>\nreferred to patching binaries,\nnot for removing hard-coded breakpoints after they stopped\nin the debugger.<\/p>\n","protected":false},"excerpt":{"rendered":"
You may hear an old-timer developer use the verb zap. That proposed fix will work. Until everybody gets the fix, they can just zap the assert. The verb to zap means to replace a breakpoint instruction with an appropriate number of NOP instructions (effectively ignoring it). The name comes from the old Windows 2.x kernel […]<\/p>\n","protected":false},"author":1069,"featured_media":111744,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"footnotes":""},"categories":[1],"tags":[25,105],"class_list":["post-15173","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-oldnewthing","tag-code","tag-microspeak"],"acf":[],"blog_post_summary":"
You may hear an old-timer developer use the verb zap. That proposed fix will work. Until everybody gets the fix, they can just zap the assert. The verb to zap means to replace a breakpoint instruction with an appropriate number of NOP instructions (effectively ignoring it). The name comes from the old Windows 2.x kernel […]<\/p>\n","_links":{"self":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/15173","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/users\/1069"}],"replies":[{"embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/comments?post=15173"}],"version-history":[{"count":0,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/15173\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/media\/111744"}],"wp:attachment":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/media?parent=15173"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/categories?post=15173"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/tags?post=15173"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}