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Text File | 1996-01-28 | 35KB | 894 lines

# ------------------------------------------------------------------- # BLITTER (c) Copyright 1996 Nat! & KKP # ------------------------------------------------------------------- # These are some of the results/guesses that Klaus and Nat! found # out about the Jaguar with a few helpful hints by other people, # who'd prefer to remain anonymous. # # Since we are not under NDA or anything from Atari we feel free to # give this to you for educational purposes only. # # Please note, that this is not official documentation from Atari # or derived work thereof (both of us have never seen the Atari docs) # and Atari isn't connected with this in any way. # # Please use this informationphile as a starting point for your own # exploration and not as a reference. If you find anything innacurate, # missing, needing more explanation etc. by all means please write # to us: # nat@zumdick.rhein-main.de # or # kkp@gamma.dou.dk # # If you could do us a small favor, don't use this information for # those lame flamewars on r.g.v.a or the mailing list. # # HTML soon ? # ------------------------------------------------------------------- # $Id: blitter.txt,v 1.11 1996/01/28 20:23:19 nat Exp $ # ------------------------------------------------------------------- Incomplete. Be wary that some stuff which sounds like we know what we're talking about, might be just a guess w/o a check :) The BLiTTER ----------- The Blitter is a little different to what you're used to on your ST (and you probably didn't get used to it very much anyway). You can blit a scaled pixmap to an unscaled destination or you can blit an unscaled pixmap unto a scaled destination. Or you can rotate the source and the destination bitmap, and in some cases you can scale and rotate at the same time (I think scaling up and rotating without leaving holes isn't possible) The former will probably be the most often used. The source or the destination can be arbitrarily 'angled' lines and need not be contigous addresses. Furthermore you can blit pixels of 1 bit, 2 bit 4 bit 8 bit 16 bit or 32bit depth. The Blitter in broad outline works like this: The blitter has two channels called A1 and A2, where it reads from and writes data to. A1 is the sophisticated channel allowing fractional pixel treatment (like f.e. read pixel 1 twice, then pixel2 twice etc. for an effective scaling of 2.0), whereas A2 is a simple channel allowing only integer increments of the addresses. This means that A2 can only be used for straight or diagonal lines. Picture in your mind that a channel is pointing to a square bitmap. You define the width of this bitmap and the origin at which the blitter should start fetching data. The origin might for example be the center of your bitmap, or the upper left corner, you decide!. You then define the orientation (slope) of the line the blitter should 'draw' into (or 'fetch' from) this bitmap. .....width....... channel --> +----------------+ | | | x (origin) | | \ | | \ (slope) | | | +----------------+ In a real life environment you might for example use A2 as the source of your texture, that is stored as a contigous block in memory and A1 is used to draw an arbitrary scaled and angled line of your polygon. Or you might use A1 to traverse the texture data at an arbitrary angle and update the destination pixmap in a scanline fashion horizontally left to right. If you want to scale the bitmap you gotta figure out whether you want to shrink or to enlarge. If you want to enlarge, you need to use A1 as the source and A2 as the destination, using fractional incrementing on the source, if you want to shrink you want to use A1 as the destination (also with fractional increments) You can do a few operations concurrently while blitting your data. If you're drawing with a single color and outputting crycolor pixels you can gouraudshade them at the same time at no extra cost. The blitter will use the intensity of the pixel and add the contents of a register to it (saturating add). The contents of this register are then updated for the next pixel. Since the update is fractional, you can achieve a smooth shaded line with this. You can add an intensity factor to your incoming crycolor data. *********************************************************************** Z-BUFFER DOC IS WRITTEN WITH NO KNOWLEDGE ABOUT THE SUBJECT WHATSOEVER *********************************************************************** You can also use the Z-buffering capabilities of the blitter. Your destination data is not just an array of pixel values, but rather a combination of Z-data and pixel data. Consider the Z-data to be the third coordinate providing depth. The smaller the value, the nearer it is to the viewer. (usual convention) You set up the blitter with a starting Z-value for your line and a factor that should be added for every pixel step, thereby possibly increasing or decreasing the Z-position. That value is then compared to the Z-data of the destination pixel. If the z-value (in the registers) is less than the the destination value, the pixel will be written - else the pixel will not be written. The destination pixel is then updated with the new Z-buffer value. The Z-data is 8 bit large ? Unfortunately Z-buffering in detail is still unknown. You can probably do collision detection on background colors, and transparent blits... Phrasemode and Pixelmode. ------------------------ The blitter can operate either in pixelmode or in phrasemode. Phrasemode is (in 16-bit crycolor) usually four times faster and is therefore much more desirable. But there are some limitations that are connected to phrasemode: o Both A1 and A2 must work in phrasemode, you can't have one running in pixelmode and the other in phrasemode o Phrasemode implies linear address (or horizontally oriented) blits It looks like phrasemode doesn't work with all resolutions. So far only 16bit modes are known to work. Scales and rotates aren't possible in phrasemode. [ Please note, that you can do (non rotated) sprite scaling also with an OP object, which might (or not) be more convenient ] There's probably something wrong with my understanding of the blitter, because I have a problem with the source DMA channel of the blitter. Currently you gotta figure, that the machine takes time = 1 write + [4 cyles read source] + [1 cycle read destination] regardless of pixelmode or phrasemode. Of course in phrasemode you usually speed up the blitting process in 16 bit pixel mode by a factor of four. Maybe the delay in the read is needed to get the ROM timings (32bit organized and maybe needing one waitstate) correct. If this is rite then there should be a bit somewhere to turn this off. But where is it ? It seems not to be in the CMD register. But if the above is true this means, that the Blitter is capable of doing approximately about: Gouraud pixelmode : 13.3 / 1 = 13.3 Mio pixels / second or ca. 222000 pixels / frame Copyblit pixelmode : 13.3 / 5 = 2.7 Mio pixels / second or ca. 44000 pixels / frame XOR blit pixelmode : 13.3 / 6 = 2.2 Mio pixels / second or ca. 37000 pixels / frame and in phrasemode (16 bit pixels) Gouraud phrasemode : 13.3 * 4 / 1 = 53.2 Mio pixels / second or ca. 887000 pixels / frame Copyblit pixelmode : 13.3 * 4 / 5 = 2.7 Mio pixels / second or ca. 177000 pixels / frame XOR blit pixelmode : 13.3 * 4 / 6 = 2.2 Mio pixels / second or ca. 148000 pixels / frame B_CMD: ---------- 32 28 24 20 16 12 8 4 0 +-+------^--------+^--------+^------+-^-----+--^--+-----^----+---^--+-----+ | | control | OP | z-op | ity | mode|A1ctl|misc| dst | src | +-+---------------+---------+-------+-------+-----+-----+----+------+-----+ 30..25 24...21 20..18 17..14 13.11 10..8 7..6 5..3 2..0 Writing into the lower word activates the blitter! Reading this register gives the blitter status (bit #0 == blitter idle if set) src: bit 0: SRCEN source data read enable bit 1: SRCENZ source Z-data read enable bit 2: SRCENX source extra data read enable With this set of bits you tell the blitter what kind of data accesses it needs to perform. It can not figure it out from the way the other command bits are set and conclude what it needs to do, you have to instruct the blitter yourself. If you're doing straight copies from memory to memory, you will want to set bit0. If you're using the Z-buffer capabilities you'd want to set bit1 as well. If your source data spans more phrases than the destination data then you need to set bit2 to tell the blitter to do that extra phrase read. dst: bit 3: DSTEN destination data read enable bit 4: DSTENZ destination Z-data read enable bit 5: DSTWRZ destination Z write enable You'd want to set bit3, if you're doing read-write-modify cycles on the destination, say f.e. you're doing DST &= SRC or somesuch. Else you should clear this bit (or pay the price in speed decrease). Likewise if you're not going to do Z-buffer blitting keep bits 4 and 5 clear, else set'em! Note that you can not disable 'destination write' because, you'd just not use the Blitter in this case, right ? misc: bit 6: CLIP_A1 enable A1 clipping bit 7: unused You can clip the pixmap that is handled with the A1 register set. If this bit is set, then the information in the A1_CLIP register is used to clip the A1 lines. See A1_CLIP for more information about clipping. A1-control (A1ctl): bit 8: UPDA1 enable A1 update step fraction part bit 9: UPDA1F enable A1 update step integer part bit10: UPDA2 enable A2 update step You hint the Blitter here, which step registers it should update. If you're just doing linedrawings you don't need any of these bits set. Only when you're blitting in two dimensions you need to consider these bits. The idea behind them is probably not that you can improve the blitter performance but rather the setup performance, since you know which registers change and which not and need not update all of them for consecutive blits. mode: bit11: DSTA2 use A2 as destination bit12: GOURD enable Gouraud shading bit13: ZBUFF enable Z-buffer handling Usually (bit11 cleared) you use A2 as the source and A1 as the destination. You can reverse the roles by setting this bit. Set bit12 to enable Gouraudshading. Gouraudshading will only be "gouraud shading" if used on crycolor data. Use the intensity counters/incrementers to specify the shading (see B_IINC for further reference) With bit13 you enable Z-buffer handling (look for the A1_FLAGS for a small description of Z-buffer handling). intensity (ity) and other stuff: bit14: TOPBEN carry into nybble bit15: TOPNEN carry into byte bit16: PATDSEL use pattern data (instead of source) Bit14 and 15 will all be explained in the gouraud shading description coming up soon. You can control with the bit 14 + 15 where the overflow from the intensity addition should be stored (added to) On a completely different note, if you just want to initialize a memory region (or draw a line) in a single color, you don't need to read the source data from memory. You can let the blitter pull the color from one of its own registers (B_PATD). This saves you on the average a read cycle for every phrase written, which is a good thing. None of the logical blitter operations apply when using the pattern data register. You can't XOR your bitmap with the pattern data! z-op: bit18-20: bit18: ZMODELT source < destination bit19: ZMODEEQ source = destination bit20: ZMODEGT source > destination or 0: unused 1: src < dst 2: src == dst 3: src <= dst 4: src > dst 5: src != dst 6: src >= dst 7: unused You can tell the blitter how to decide, whether the source data should overwrite the destination pixel or not when using the Z-buffer mode. Usually you will want to put a 3 or a 1 here, so that you're 'nearer' pixels overwrite the 'farther' pixels. (Assuming that your Z-buffer values are the higher, the farther away from the viewer they are) OP: logical operation the Blitter should perform bit21: LFU_NAN ! source & ! destination bit22: LFU_NA ! source & destination bit23: LFU_AN source & ! destination bit24: LFU_A source & destination or 0: LFU_ZERO DST = 0 (LFU_CLEAR) 1: LFU_NSAND DST = ! SRC & ! DST 2: LFU_NSAD DST = ! SRC & DST 3: LFU_NOTS DST = ! SRC 4: LFU_SAND DST = SRC & ! DST 5: LFU_NOTD DST = ! DST 6: LFU_N_SXORD DST = ! (SRC ^ DST) 7: LFU_NSORND DST = ! SRC | ! DST 8: LFU_SAD DST = SRC & DST 9: LFU_SXORD DST = SRC ^ DST (LFU_XOR) 10: LFU_D DST = DST 11: LFU_NSORD DST = ! SRC | DST 12: LFU_S DST = SRC (LFU_REPLACE) 13: LFU_SORND DST = SRC | ! DST 14: LFU_SORD DST = SRC | DST 15: LFU_ONE DST = 1 Just as on the Atari ST blitter you can have the usual set of logical operations you can perform on your data. Use 12 for your copying blits and 0 for your single color initilization. Note that if you set bit16 (use pattern data), then the blitter will NOT zero your buffer with OP==0, but fill it with the pattern color instead. The opcodes are ignored when bit16 is set. control: bit25: CMPDST compare destination pixel with pattern pixel bit26: BCOMPEN bit compare write inhibit bit27: DCOMPEN data compare write inhibit bit28: BKGWREN unknown bit29: BUSHI hog the bus bit30: SRCSHADE source shading bit25: If you enable this the destination pixel (that will be overwritten) is compared with the value stored in the pattern-data register (B_PATD). If you enable this in conjuction with B_STOP this _maybe_ is used as a way to do hardware collision detection. (like in GTIA on the Atari 8 bit) bit26: speculation: The lower 8 bit of the source value are examined. If all bits are 0 then nothing will be written back, if all of them are set then everything will be written back. Now what happens if there are just a few bits set ? Imagine that the pixels of the destination pixmap are numbered from 7 to 0 wrapping at -1 back to 8. Start of line blit \ 7654321076543210765 ^ current pixel position Source pixel value: 0xFF55 -> 11111111 01010101 76543210 01010101 ^ So this pixel value will not be written. Don't ask me what that might be good for. bit27: used in conjunction with bit25. If you set bit25 and bit27 the effect will be that only those destination values will be overwritten that do not match the value stored in B_PATD. So if you put the color 0x0000 into B_PATD only those pixels will be written, where there are not zerovalued pixels in the destination bitmap. You should have DSTEN on! bit28: still no idea yet bit29: seems to let the blitter hog the bus completely. This is not such a good idea for extensive blits, since apparently the OP is also shut off and you'll see garbage on the screen. For small blits this might yield an overall system performance increase, when you're pushing the machine to its limits. bit30: Enable source shading. Yes it does work, although the setup is a bit weird because you seem to have to set bit3 (destination read enable) for real source shading to happen. Put the shade value into B_IINC. Looks really cool. You can get some funky albeit as yet unpredictable (?) effects putting a value in B_DSTD and disabling the destination read. F.e. put the B_IINC to $40000 and blit repeatedly incrementing B_DSTD (and delaying a little between blits). It's psychedelic! B_COUNT: -------- 32 28 24 20 16 12 8 4 0 +--------^---------^---------^--------+--------^--------^--------^--------+ | n_lines | n_pixels | +-------------------------------------+-----------------------------------+ 16 bit 16 bit n_pixels: number of pixels to draw in a line n_lines: number of lines to draw You need to draw at least one line of size one pixel. After n_pixels are drawn the STEP registers are applied to the current pixel position and blitting resumes. B_IINC + B_I0-3: --------------- 32 28 24 20 16 12 8 4 0 +--------+---------+---------^--------+--------^--------^--------^--------+ |chroma.i|chroma.f | intensity.i | intensity.f | +--------+---------+------------------+-----------------------------------+ 4 bit 4 bit 8 bit 16 bit chroma.i: delta for chroma change, integer part chroma.f: delta for chroma change, fractional part intensity.i delta for gouraud shading, integer part intensity.f: delta for gouraud shading, fractional part This register is used for chroma changes and gouraud shading (or both operations together). Chroma changes are like gouraud shades, but with an intensity delta of zero. Pure gouraud shadings have a chroma delta value of zero. This register is added to either B_PATD in pixelmode or B_I0 to B_I3 in phrasemode. The intensity is saturation added, meaning that you can't have an intensity wrap around. The chroma change on the other hand does wrap around. The integer part is by the way sign extended. So normally chroma and intensity are two seperate entities that don't influence each other. If you want you can set in the B_CMD register either TOPBEN or TOPNEN. If you set TOPNEN then the carry of the saturation add will be added to the upper nyblle (chroma.i) of the current source data value. If you set TOPBEN then there will be _no_ saturation for the addition of the intensity delta. Instead the carry is added to the top byte of the current source data value. B_I0, B_I1, B_I2, B_I3 are used in phrasemode instead of B_IINC, which used in pixelmode. B_DSTD: ------ 32 28 24 20 16 12 8 4 0 +--------^---------^---------^--------^--------^--------^--------^--------+ 0 | pixelvalue | +-------------------------------------------------------------------------+ 64 60 56 52 48 44 40 36 32 +--------^---------^---------^--------^--------^--------^--------^--------+ 1 | pixelvalue | +-------------------------------------------------------------------------+ pixelvalue: If you're doing RMW-cycles with the blitter and have not enabled data reads, then this register will be used as input for the logical operations instead. Depending on the blittermode (pixelmode or phrasemode) there is either only one pixel kept in here (phrase 0) rightside aligned by the way, or as many pixels that can fit in a phrase. Experiments show that the value in DSTD is NOTed before being used as a logical operation. Curious. f.e. move.l #(1<<16)|WIDTH,B_COUNT move.l #PITCH1|PIXEL16|WID320|XADDPIX,A1_FLAGS move.l #PITCH1|PIXEL16|WID320|XADDPIX,A2_FLAGS move.l #$00000FFFF,B_DSTD move.l #SRCEN|LFU_XOR,d0 is actually a straight replacement, although one would expect S ^ 0xFFFF to yield ~S and not S B_SRCD: ------ 32 28 24 20 16 12 8 4 0 +--------^---------^---------^--------^--------^--------^--------^--------+ 0 | pixelvalue | +-------------------------------------------------------------------------+ 64 60 56 52 48 44 40 36 32 +--------^---------^---------^--------^--------^--------^--------^--------+ 1 | pixelvalue | +-------------------------------------------------------------------------+ pixelvalue: This is probably just the same as B_DSTD but for those case when you did not have source read enabled (bit #0 of the CMD register) and when you haven't selected the pattern as the source of your blit. B_STOP: ------ 32 28 24 20 16 12 8 4 0 +--------^---------^---------^--------^--------^--------^--------^-+------+ | unused | stop | +------------------------------------------------------------------+------+ stop: bit 0: bit 1: bit 2: Uses 3 bits to resume or stop after a write inhibit occures. Inhibit will occure when painting pixel-pixel mode, Xadd=1, BKGWREN=0, and one of the BCOMPEN, DCOMPEN or Zmodem0-2 are set, with matching conditions. ????????????????????????? A1_BASE: ------- 32 28 24 20 16 12 8 4 0 +--------^---------^---------^--------^--------^--------^--------^--------+ | address | +-------------------------------------------------------------------------+ address: Pointer to the bitmap. The bitmap must (probably) be phrase aligned. For pixel positioning use A1_PIXEL A1_FLAGS: -------- 32 28 24 20 16 12 8 4 0 +--------^---------^--------+---------+--+-----^-----+--^---+----^-+------+ | unused | addctl | | width | z-off| depth| pitch| +---------------------------+---------+--+-----------+------+------+------+ 20...16 14...9 8..6 5..3 2..0 ^ pitch: +---- unused bit0-bit2: 0: 1 phrase 1: 2 phrases 2: 4 phrases 3: 8 phrases The amount of phrases the blitter should add to the address when accessing the next phrase. Usually set to zero eh ? depth: bit3-bit5: colors bitplanes bits ------------+-------------------+------------- 0: 2 1 1 1: 4 2 2 2: 16 4 4 3: 256 8 8 4: 32768/65536 16/CrY 16 5: 16 mio 24 32 6: unused 7: unused The pixel size the blitter should move. Remember all pixels on the Jaguar are chunky (meaning the bits to a pixel are adjacent, not like on the Amiga or the ST) z-offset (z-off): bit6-bit8 gives the number of phrases the Z-data is offset from your pixel phrase. Or so... Apparently 0 and 7 are unusable values width: bit9-14: This is the width in pixels of a scanline of the area pointed to by A1. Or in different words A1 points to a rectangular block of pixels. The pixels are organized in horizontal strips. You give the width of such a strip with this value. The number is not an integer value but rather a floating point value (no kidding). It is made up like this: 1.[bit14-13] * 2^[bit12-9] so for example 01 0101 would be 1.25 * 2^5 = 40 or 10 1000 would be 1.5 * 2^8 = 384 (1.00bin -> 1.00dec 1.01bin -> 1.25dec 1.10bin -> 1.5dec 1.11bin -> 1.75dec) or you can think of it as: x = 1 << [bit12-9] res = x + (bit14 ? (x >> 1) : 0) + (bit15 ? (x >> 2) : 0); 01 0101 would be x = 1 << 5 /* 32 */ res = 32 + (0 ? 16 : 0) + (1 ? 8 : 0); /* 40 */ Some often used values are: value width value width value width -------+------- -------+------- -------+------- 4 2 8 4 10 6 12 8 13 10 14 12 15 14 16 16 17 20 18 24 19 28 20 32 21 40 22 48 23 56 24 64 25 80 26 96 27 112 28 128 29 160 30 192 31 224 32 256 33 320 34 384 35 448 36 512 37 640 38 768 39 896 40 1024 41 1280 42 1536 43 1792 44 2048 45 2560 46 3072 47 3584 adding control (addctl) ** please note that the bit descriptions are as an exception ** interleaved Xadd control bit16-17: 0: XADDPHR add phrase offset to X and truncate 1: XADDPIX add pixelsize (1) to X 2: XADD0 add zero (for those nice vertical lines) 3: XADDINC add the contents of the increment register bit19: XSIGNADD/XSIGNSUB pixel add operation, 0 = add 1 = subtract when using "add pixelsize" mode If you don't set any of these bits (0) then you are using the blitter in phrase mode. That means that pixels are grabbed in lots of phrases updated concurrently (one step) and written back in lots of phrases. (lots as in "quantitysize", not as "many"). Obviously you can use the phrasemode only for horizontal line blitting operations. Else you need to put the Blitter in pixel mode (in CryMode ~4x slower). Yadd control bit18: YADD0/YADD1 add zero (clear) or one (set) to Y bit20: YSIGNADD/YSIGNSUB add 1/sub1 to Y (when bit18 is set) A1_CLIP: -------- 32 28 24 20 16 12 8 4 0 +--------^---------^---------^--------+--------^--------^--------^--------+ | height | width | +-------------------------------------+-----------------------------------+ Height is the height and the width that the blitter should clip at starting from the base. It does work but seems to be buggy. (i.e. sometimes clips one pixel to early) A1_PIXEL: -------- 32 28 24 20 16 12 8 4 0 +--------^---------^---------^--------+--------^--------^--------^--------+ | Y.i | X.i | +-------------------------------------+-----------------------------------+ X.i: horizontal position (integer part) Y.i: likewise vertical pixel offset horizontal and vertical pixel offset from A1_BASE where the blitting operation should begin. Note that for the calculation of the proper address offset, the blitter needs to know the pixel size and the width of one line A1_FPIXEL: --------- 32 28 24 20 16 12 8 4 0 +--------^---------^---------^--------+--------^--------^--------^--------+ | Y.f | X.f | +-------------------------------------+-----------------------------------+ X.i: horizontal position (fractional part) Y.i: likewise vertical pixel offset You can position the pixel value at a fractional pixel value using this register. If you're using fractional steping rates, this register will be updated as well as the integer A1_PIXEL register. Guess: 0.FFFF will still address pixel 0 and will not round up to 1. A1_INC: ------ 32 28 24 20 16 12 8 4 0 +--------^---------^---------^--------+--------^--------^--------^--------+ | unused | increment.i | +-------------------------------------------------------------------------+ increment.i: integer delta added to pixel position offset to add to A1_PIXEL after a pixel has been blitted. this register is used only, if a certain addressing mode (bit16... of A1_FLAGS) is used. Please also note the update A1 bits in the blitter command for proper operation. The size is not yet known, but believed to be 16 bit A1_FINC: ------- 32 28 24 20 16 12 8 4 0 +--------^---------^---------^--------+--------^--------^--------^--------+ | unused | increment.f | +-------------------------------------+-----------------------------------+ increment.f: fractional delta added to pixel position As above but this is the fractional part of the stepper. A1_STEP: ------- 32 28 24 20 16 12 8 4 0 +--------^---------^---------^--------+--------^--------^--------^--------+ | y_step.i | x_step.i | +-------------------------------------+-----------------------------------+ x_step.i: value to be added to A1_PIXEL.x y_step.i: value to be added to A1_PIXEL.y Values added to the pixelpointer after a line has been drawn. You must set a bit in the control register to allow this update to happen. A1_FSTEP: -------- 32 28 24 20 16 12 8 4 0 +--------^---------^---------^--------+--------^--------^--------^--------+ | y_step.f | x_step.f | +-------------------------------------+-----------------------------------+ x_step.f: value to be added to A1_FPIXEL.x y_step.f: value to be added to A1_FPIXEL.y Values added to the pixelpointer after a line has been drawn. You must set a bit in the control register to allow this update to happen, which is different than the control bit used for integer step updates! A2_BASE: ------- 32 28 24 20 16 12 8 4 0 +--------^---------^---------^--------^--------^--------^--------^--------+ | address | +-------------------------------------------------------------------------+ See A1_BASE A2_FLAGS: -------- 32 28 24 20 16 12 8 4 0 +--------^---------^--------+^--------+--+-----^-----+--^---+----^-+------+ | unused | addctl | | width | z-off| depth| pitch| +---------------------------+---------+--+-----------+------+------+------+ 20...16 14...9 8..6 5..3 2..0 See A1_FLAGS A2_PIXEL: -------- 32 28 24 20 16 12 8 4 0 +--------^---------^---------^--------+--------^--------^--------^--------+ | Y.i | X.i | +-------------------------------------+-----------------------------------+ See A1_PIXEL A2_MASK: ------ 32 28 24 20 16 12 8 4 0 +--------^---------^---------^--------+--------^--------^--------^--------+ | mask.y | mask.x | +-------------------------------------+-----------------------------------+ mask.x: x modulo value mask.y: y modulo value A2_MASK is probably used to mask off the pixel value of A2 creating thereby a circular buffer A2_STEP: ------- 32 28 24 20 16 12 8 4 0 +--------^---------^---------^--------+--------^--------^--------^--------+ | y_step.i | x_step.i | +-------------------------------------+-----------------------------------+ See A1_STEP BUGS: ----- It seems that when the blitter is done with blitting it is so happy that it's done, it forgets to update the PIXEL registers with the STEP registers (UPDA1 UPDA2 UPDA1F). Therefore this: move.l #$00400040,B_COUNT move.l #VALUE,B_CMD is not equivalent to this moveq #$3F,d1 .loop: move.l #$00010040,B_COUNT move.l #VALUE,B_CMD .wait: move.l B_CMD,d0 ror.w #1,d0 bcc.b .wait dbra d1,.loop Clipping seems to be buggy, occasionally clipping too early. (??) DISCUSSION: ---------- The designers of the Jaguar thought a lot about what makes a good blitter and not. Important for a good blitter is that the setup time is minimal. One of the reasons the Atari ST Blitter wasn't that successful was that in the time needed to setup the chip - and you did have to setup and calculate quite a lot of values - you were typically just about done doing the blit in software (a small exaggeration). The Jaguar blitter needs minimal setting up, because the hardware does a lot of the calulations for you. For doing a memory clear f.e. you need to write only five registers: A1_BASE A1_PIXEL A1_FLAGS A1_CMD B_PATD With a little imagination you will find a lot of uses for this nice piece of hardware, that go way outside of just drawing and filling.