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Diffstat (limited to 'xex.rst')
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@@ -80,9 +80,119 @@ exit. The run address segment is the one that loads the 2-byte run address into address **$02E0**, aka **RUNAD**. +Raw Data Blocks +--------------- + +When init code runs, the file is still being loaded. It's open on IOCB +#1, and the init code can read some or all of the rest of the file +itself, before returning to DOS. The part of the file read by an init +segment doesn't have to conform to the regular XEX file structure, so +they're referred to here as raw data blocks. + +Raw data blocks usually occur in files created with "packer" or +"compressor" programs, or occasionally in other large programs (Turbo +BASIC is an example). Raw data blocks are generally found just after +an init address segment. If you have an executable that loads just +fine on a real Atari or emulator, but fails with **xexcat** and +**xexamine**, a raw data block is usually the reason why. + +Files with raw data blocks also can't be loaded by simple bootloaders +such as Fenders 3-sector loader. + EXAMPLES ======== +Assembly +--------- + +Here is a simple assembly language program that changes the background +of the GRAPHICS 0 text screen to black, and the text to high-intensity white:: + + 1000 *= $0600 + 1010 START + 1020 LDA #0 + 1030 STA 710 + 1040 LDA #$0F + 1050 STA 709 + 1060 RTS + 2000 *= $02E0 ; AKA RUNAD + 2010 .WORD START + +To assemble this with either Atari's Assembler/Editor or OSS Mac/65, +the command is:: + + ASM ,,#D:COLORS.XEX + +This will create a binary file that looks like this:: + + offset 0: FF FF 00 06 0A 06 A9 00 + offset 8: 8D C6 02 A9 0F 8D C5 02 + offset 16: 60 E0 02 E1 02 00 06 + +This is a complete (though short) XEX file, and it can be loaded from +the DOS menu (or D1: prompt, if you use a command-line-based DOS). + +The first 2 bytes ($FF, $FF) are the signature for the initial +segment header. + +The next 2 bytes ($00, $06) are the load address of the first segment +($0600, in 6502-style LSB-first notation). + +The next 2 bytes ($0A, $06) are the end address ($060A). + +Since the header says to load data from $0600 to $060A, there are 11 +data bytes in the segment, beginning with $A9, $00 (the 6502 object +code for the *LDA #0* instruction), and extending to the $60 (RTS +opcode) at offset 16. + +The data from the first segment is immediately followed by the header +of the next segment, at offset 17. A $FF, $FF signature would be allowed +here, but in the example, the 2nd segment uses the 4-byte header. + +At offset 17, the $E0, $02 (aka $02E0) is the load address. $E1, $02 +($02E1) is the end address. $02E0/$02E1 is known as *RUNAD* in the +Atari world, and it's the address where DOS will find the entry point +to the program when it's done being loaded. + +The next (and last) 2 bytes are $00, $06 (aka $0600), which is the run +address itself (to be deposited at *RUNAD*). + +There are no more segments, since we've reached end of file. + +Data Only +--------- + +Since a XEX file can load arbitrary data at arbitrary addresses, there's +simpler way to accomplish the color changes. Instead of writing code to +do the job, we just create a XEX file that loads the colors we want +directly into the color shadow registers. + +In BASIC, you'd say:: + + POKE 709,15 + POKE 710,0 + +...but BASIC doesn't have an easy way to create a XEX file. +An assembler is a more convenient tool. The code is:: + + 1000 *= $02C5 + 1010 .BYTE $0F,$00 + +The binary looks like this:: + + ff ff C5 02 C6 02 0F 00 + +The file begins with $FF, $FF header, then the 2-byte load address +($C5, $02 for address $02C5), then the 2-byte end address ($C6, $02, +aka $02C6), then the 2 bytes of data to be stored there ($0F, $00). + +When loaded, this will store $0F at address $02C5 (aka *COLOR0*, +the text luminance in *GRAPHICS 0*) and $00 at $02C6 (*COLOR1*, the +*GRAPHICS 0* background color). + +Notice that there's no run or init address. That's because no code is +executed; it's just 2 bytes of raw data. + HISTORY ======= |