Subject: RE: [stella] Help (Timing problems?) From: Nicolás Olhaberry <nolh@xxxxxxxxxxxxx> Date: Wed, 11 Aug 1993 04:58:12 -0300 |
----- Original Message ----- From: Wade Brown To: stella@xxxxxxxxxxx Sent: Thursday, August 09, 2001 10:18 PM Subject: Re: [stella] Help (Timing problems?) > Maybe someone out there has a link to a better opcode ref - If so I (and the list, no doubt) would sure appreciate hearing about it. I´ve download from I-don´t-remember-where a fairly complete documentation for the SY6500, which for what I know, is equivalent to the 6502. I had been using it for a while without any problem (be sure to ignore the SY65C02 part, it has newer features which aren´t available on the 6502). If you can´t find it on the web, let me know and I will email it to you. You can also try with the assembly language chapter from the C64 manual, but I don´t know if it can be found online, anyway it won´t tell you much more than the SY6500 documentation. >> bcc .inXBounds >> lda #RIGHT_BOUND >> .byte $2c ; opcode for bit.w, this hides the next instruction <-- I don't understand? (Wade) >>.outOfLeftBound: >> lda #LEFT_BOUND >> sta Player1X >>.inXBounds: I will try to explain this, but I know my english will fail me, sorry. .byte $2c is a directive for the compiler, not an opcode, so it won´t be interpreted as one, and simply $2C will be written to the binary in that position. When this code is executed, $2C will be fetched, which is the value for the opcode BIT in absolute addressing mode. That mode needs two bytes of data to determine a memory location, so, the next instruction lda #LEFT_BOUND (since it´s two bytes long), will be considered data, instead of being executed. Notice that when bcc outOfLeftBound: is taken, lda #LEFT_BOUND will be executed. The purpose of this, is to save ROM space, because just one more byte allows you to store the left or the right bound on Player1X . Bye, Nicolás. - Archives (includes files) at http://www.biglist.com/lists/stella/archives/ Unsub & more at http://www.biglist.com/lists/stella/
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