Up to now we have dealt with the blitter only in moving words of memory around and combining them with logic operations. This is sufficient for moving graphic images around, so long as the images stay in the same position relative to the beginning of a word. If our car image has its leftmost pixel on the second pixel from the left, we can easily draw it on the screen in any position where the leftmost pixel also starts two pixels from the beginning of some word. But often we want to draw that car shifted left or right by a few pixels. To this end, both the A and B DMA channels have a barrel shifter that can shift an image between 0 and 15 bits. This shifting operation is completely free; it requires no more time to execute a blit with shifts than a blit without shifts, as opposed to shifting with the 680x0. The shift is normally towards the right. This shifter allows movement of images on pixel boundaries, even though the pixels are addressed 16 at a time by each word address of the bitplane image. So if the incoming data is shifted to the right, what is shifted in from the left? For the first word of the blit, zeros are shifted in; for each subsequent word of the same blit, the data shifted out from the previous word is shifted in. The shift value for the A channel is set with bits 15 through 12 of BLTCON0 ; the B shift value is set with bits 15 through 12 of BLTCON1 . For most operations, the same value will be used for both shifts. For shifts of greater than fifteen bits, load the address register pointer of the destination with a higher address; a shift of 100 bits would require the destination pointer to be advanced 100/16 or 6 words (12 bytes), and a right shift of the remaining 4 bits to be used. As an example, let us say we are doing a blit that is three words wide, two words high, and we are using a shift of 4 bits. For simplicity, let us assume we are doing a straight copy from A to D. The first word that will be written to D is the first word fetched from A, shifted right four bits with zeros shifted in from the left. The second word will be the second word fetched from the A, shifted right, with the least significant (rightmost) four bits of the first word shifted in. Next, we will write the first word of the second row fetched from A, shifted four bits, with the least significant four bits of the last word from the first row shifted in. This would continue until the blit is finished. On shifted blits, therefore, we only get zeros shifted in for the first word of the first row. On all other rows the blitter will shift in the bits that it shifted out of the previous row. For most graphics applications, this is undesirable. For this reason, the blitter has the ability to mask the first and last word of each row coming through the A DMA channel . Thus, it is possible to extract rectangular data from a source whose right and left edges are between word boundaries. These two registers are called BLTAFWM and BLTALWM, for blitter A channel first and last word masks. When not in use, both should be initialized to all ones ($FFFF). A note about fonts. ------------------- Text fonts on the Amiga are stored in a packed bitmap. Individual characters from the font are extracted using the blitter, masking out unwanted bits. The character may then be positioned to any pixel alignment by shifting it the appropriate amount. These masks are "anded" with the source data, before any shifts are applied. Only when there is a 1 bit in the first-word mask will that bit of source A actually appear in the logic operation. The first word of each row is anded with BLTAFWM, and the last word is "anded" with BLTALWM. If the width of the row is a single word, both masks are applied simultaneously. The masks are also useful for extracting a certain range of "columns" from some bitplane. Let us say we have, for example, a predrawn rectangle containing text and graphics that is 23 pixels wide. The leftmost edge is the leftmost bit in its bitmap, and the bitmap is two words wide. We wish to render this rectangle starting at pixel position 5 into our 320 by 200 screen bitmap, without disturbing anything that lies outside of the rectangle. |<--------- two word source bitmap -------->| | | |<--- extract a 32-bit image --->| | | | | |<--- 16-bit word -->| | | |____________________|___________|__________| | | source | 00000000 00000000 00000000 00000000 | DMA B | 11111111 11111111 11111111 11111111 | | 10101010 01010101 10101010 01010101 | |___________________________________________| | | | | Source is passed through mask when it is one, outherwise the destination is copied. | | | | _____v_________v____ ___v_________v______ | | | | mask on | 11111111 11111111 | | 11111110 00000000 | DMA A | first word mask | | second word mask | |____________________| |____________________| | | | || | | | | |- - _____v_________v__________v__v_____________ final | | destination | 00000000 00000000 00000000 11111111 | DMA D | 11111111 11111111 11111111 11111111 | (points to | 10101010 01010101 10101010 11111111 | same address |___________________________________________| as DMA C) ^ ^ ^ ^ - - - | | | | <-+ _____|_________|_________|____|___|__|___|_ | destination | | | before blit | 00000000 00000000 00000000 00000000 | | DMA C | 11111111 11111111 11111111 11111111 | | (to be | 10101010 01010101 10101010 01010101 | | overwritten) |___________________________________________| | | Destination does not change where mask is 0 --+ Figure 6-4: Extracting a Range of Columns To do this, we point the B DMA channel at the bitmap containing the source image, and the D DMA channel at the screen bitmap. We use a shift value of 5. We also point the C DMA channel at the screen bitmap. We use a blit width of 2 words. What we need is a simple copy operation, except we wish to leave the first five bits of the first word, and the last four bits (2 times 16, less 23, less 5) of the last word alone. The A DMA channel comes to the rescue. We preload the A data register with $FFFF (all ones), and use a first word mask with the most significant five bits set to zero ($07FF) and a last word mask with the least significant four bits set to zero ($FFF0). We do not enable the A DMA channel , but only the B, C, and D channels, since we want to use the A channel as a simple row mask. We then wish to pass the B (source) data along wherever the A channel is 1 (for a minterm of AB) and pass along the original destination data (from the C channel) wherever A is 0 _ (for a minterm of AC), _ yielding our classic cookie-cut function of AB+AC, or $CA. About disabling. ---------------- Even though the A channel is disabled, we use it in our logic function and preload the data register . Disabling a channel simply turns off the memory fetches for that channel; all other operations are still performed, only from a constant value stored in the channel's data register . An alternative but more subtle way of accomplishing the same thing is to use an A shift of five, a first word mask of all ones, and a last word mask with the rightmost nine bits set to zero. All other registers remain the same. Warning: -------- Be sure to load the blitter immediate data registers only after setting the shift count in BLTCON0/BLTCON1 , as loading the data registers first will lead to unpredictable results. For instance, if the last person left BSHIFT to be "4", and I load BDATA with "1" and then change BSHIFT to "2", the resulting BDATA that is used is "1<<4", not "1<<2". The act of loading one of the data registers "draws" the data through the machine and shifts it.
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