{"id":2703,"date":"2022-12-15T08:31:48","date_gmt":"2022-12-15T08:31:48","guid":{"rendered":"https:\/\/cml-a.com\/content\/?p=2703"},"modified":"2022-12-15T08:31:48","modified_gmt":"2022-12-15T08:31:48","slug":"cursed","status":"publish","type":"post","link":"https:\/\/cml-a.com\/content\/2022\/12\/15\/cursed\/","title":{"rendered":"Cursed"},"content":{"rendered":"\n

Consider this 65816 program<\/p>\n\n\n\n

.cpu \"65816\"                        \n\nPUTS = $00101C                      \nPUTC = $001018                      \n* = $00FFFC\nRESET   .word <>START\n\n* = $002000\nSTART   CLC                         ; Make sure we're native mode\n        XCE\n\n        REP #$30\n        .al\n        .xl\n        JSR MSG1\n\n        SEP #$30  ; Set 8bit axy\nDIV\n\n;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;\n; Value      ; 8bit interpretation    ; 16bit interpretation\n;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;\n;            ;                        ;\n.byte $A9    ; LDA #$3A               ; LDA #$3A3A                    \n.byte $3A    ;                        ;\n.byte $3A    ; DEC A                  ;       \n;            ;                        ;        \n.byte $29    ; AND #$39               ; AND #$3A39         \n.byte $39    ;                        ;   \n;            ;                        ;      \n.byte $3A    ; DEC A                  ;       \n;            ;                        ;      \n.byte $29    ; AND #$38               ; AND #$2038       \n.byte $38    ;                        ;   \n;            ;                        ;      \n.byte $20    ; JSR $20EA              ;                                    \n.byte $EA    ;                        ; NOP\n;            ;                        ;      \n.byte $20    ;                        ; JSR $20E0\n;            ;                        ;      \n.byte $E0    ; \n.byte $20    ; \n\n        TAX\n        JSR CLRB\n        JSL PUTS \n        JSR MSG2\n\nDONE    NOP         ; Spin\n        BRA DONE\n\n* = $002038\nMODE16 .null \"16\"\nPRE   .null \"This is in \"\nSUF   .null \"-bit mode.     \"\n\nCLRB    LDA #$0000\n        PHA\n        PLB\n        PLB\n        RTS\n\nMSG1    JSR CLRB\n        LDX #<>PRE\n        JSL PUTS \n        RTS\n\nMSG2    JSR CLRB\n        LDX #<>SUF\n        JSL PUTS \n        RTS\n\n* = $0020E0\n        RTS\n\n* = $0020EA\n        JSL PUTC\n        REP #$30\n        .al\n        .xl\n        JSR MSG2\n        JSR MSG1\n        JMP DIV<\/code><\/pre>\n\n\n\n
 for C256 Foenix, assembled with 64tass<\/a>.<\/p>\n\n\n\n
When run, the output looks like<\/p>\n\n\n\n
\"\"<\/figure>\n\n\n\n
Explanation: the part of the program labeled 'DIV' will run twice, under different interpretations. First in 8bit mode, then in 16bit mode, for displaying the '8' and '16' printable characters respectively.  <\/p>\n\n\n\n
Normally code in 8bit is garbage when interpreted in 16bit mode and vice-versa. These ops were specially chosen so that they are valid in both with different behavior<\/p>\n\n\n\n
Because it's not possible to express the reinterpretation idea in an assembly language, this just dumps the code bytes in the middle of the program and there's 2 columns of commented-out language explaining what the bytes do. The 2 columns are one for 8bit and one for 16bit.<\/p>\n\n\n\n
I wrote it as a silly test. It's a test for the debugger. It's to see how it might display in a 'source-style' debugger. When running it I pass the debugger my source file listing. <\/p>\n\n\n\n
It goes... not great<\/p>\n\n\n\n
'Transcript debugging' described in this earlier post<\/a> fixes it, it's 100% coherent  and matches the source. <\/p>\n\n\n\n
\"\"<\/a><\/figure>\n\n\n\n
So a good vote to use that kind of debugger for this type of thing.<\/p>\n\n\n\n
Full source code available here:<\/p>\n\n\n\n
https:\/\/github.com\/clandrew\/experiments\/blob\/main\/div\/div.s<\/a><\/p>\n","protected":false},"excerpt":{"rendered":"
Consider this 65816 program for C256 Foenix, assembled with 64tass. When run, the output looks like Explanation: the part of the program labeled ‘DIV’ will run twice, under different interpretations. First in 8bit mode, then in 16bit mode, for displaying the ‘8’ and ’16’ printable characters respectively. Normally code in 8bit is garbage when interpreted […]<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[1],"tags":[4,178,200],"class_list":["post-2703","post","type-post","status-publish","format-standard","hentry","category-uncategorized","tag-4","tag-software-development-project","tag-tools"],"_links":{"self":[{"href":"https:\/\/cml-a.com\/content\/wp-json\/wp\/v2\/posts\/2703","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/cml-a.com\/content\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/cml-a.com\/content\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/cml-a.com\/content\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/cml-a.com\/content\/wp-json\/wp\/v2\/comments?post=2703"}],"version-history":[{"count":0,"href":"https:\/\/cml-a.com\/content\/wp-json\/wp\/v2\/posts\/2703\/revisions"}],"wp:attachment":[{"href":"https:\/\/cml-a.com\/content\/wp-json\/wp\/v2\/media?parent=2703"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/cml-a.com\/content\/wp-json\/wp\/v2\/categories?post=2703"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/cml-a.com\/content\/wp-json\/wp\/v2\/tags?post=2703"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}