--- title: Michael Abrash's Graphics Programming Black Book, Special Edition author: Michael Abrash date: '1997-07-01' identifier: - scheme: ISBN text: 1576101746 publisher: The Coriolis Group category: 'Web and Software Development: Game Development,Web and Software Development: Graphics and Multimedia Development' chapter: '22' pages: 413-420 --- ## Chapter 22 -- Zenning and the Flexible Mind ### Taking a Spin through What You've Learned And so we come to the end of our journey; for now, at least. What follows is a modest bit of optimization, one which originally served to show readers of *Zen of Assembly Language* that they had learned more than just bits and pieces of knowledge; that they had also begun to learn how to apply the flexible mind—unconventional, broadly integrative thinking—to approaching high-level optimization at the algorithmic and program design levels. You, of course, need no such reassurance, having just spent 21 chapters learning about the flexible mind in many guises, but I think you'll find this example instructive nonetheless. Try to stay ahead as the level of optimization rises from instruction elimination to instruction substitution to more creative solutions that involve broader understanding and redesign. We'll start out by compacting individual instructions and bits of code, but by the end we'll come up with a solution that involves the very structure of the subroutine, with each instruction carefully integrated into a remarkably compact whole. It's a neat example of how optimization operates at many levels, some much less determininstic than others—and besides, it's just plain fun. Enjoy! ### Zenning In Jeff Duntemann's excellent book *Borland Pascal From Square One* (Random House, 1993), there's a small assembly subroutine that's designed to be called from a Turbo Pascal program in order to fill the screen or a systemscreen buffer with a specified character/attribute pair in text mode. This subroutine involves only 21 instructions and works perfectly well; however, with what we know, we can compact the subroutine tremendously and speed it up a bit as well. To coin a verb, we can "Zen" this already-tight assembly code to an astonishing degree. In the process, I hope you'll get a feel for how advanced your assembly skills have become. Jeff's original code follows as Listing 22.1 (with some text converted to lowercase in order to match the style of this book), but the comments are mine. **LISTING 22.1 L22-1.ASM** ```nasm OnStack struc ;data that's stored on the stack after PUSH BP OldBP dw ? ;caller's BP RetAddr dw ? ;return address Filler dw ? ;character to fill the buffer with Attrib dw ? ;attribute to fill the buffer with BufSize dw ? ;number of character/attribute pairs to fill BufOfs dw ? ;buffer offset BufSeg dw ? ;buffer segment EndMrk db ? ;marker for the end of the stack frame OnStack ends ; ClearS proc near push bp ;save caller's BP mov bp,sp ;point to stack frame cmp word ptr [bp].BufSeg,0 ;skip the fill if a null jne Start ; pointer is passed cmp word ptr [bp].BufOfs,0 je Bye Start: cld ;make STOSW count up mov ax,[bp].Attrib ;load AX with attribute parameter and ax,0ff00h ;prepare for merging with fill char mov bx,[bp].Filler ;load BX with fill char and bx,0ffh ;prepare for merging with attribute or ax,bx ;combine attribute and fill char mov bx,[bp].BufOfs ;load DI with target buffer offset mov di,bx mov bx,[bp].BufSeg ;load ES with target buffer segment mov es,bx mov cx,[bp].BufSize ;load CX with buffer size rep stosw ;fill the buffer Bye:mov sp,bp ;restore original stack pointer pop bp ; and caller's BP ret EndMrk-RetAddr-2 ;return, clearing the parms from the stack ClearS endp ``` The first thing you'll notice about Listing 22.1 is that `ClearS` uses a `REP STOSW` instruction. That means that we're not going to improve performance by any great amount, no matter how clever we are. While we can eliminate some cycles, the bulk of the work in `ClearS` is done by that one repeated string instruction, and there's no way to improve on that. Does that mean that Listing 22.1 is as good as it can be? Hardly. While the speed of `ClearS` is very good, there's another side to the optimization equation: size. The whole of `ClearS` is 52 bytes long as it stands—but, as we'll see, that size is hardly set in stone. Where do we begin with `ClearS`? For starters, there's an instruction in there that serves no earthly purpose—`MOV SP,BP`. SP is guaranteed to be equal to BP at that point anyway, so why reload it with the same value? Removing that instruction saves us two bytes. Well, that was certainly easy enough! We're not going to find any more totally non-functional instructions in `ClearS`, however, so let's get on to some serious optimizing. We'll look first for cases where we know of better instructions for particular tasks than those that were chosen. For example, there's no need to load any register, whether segment or general, through BX; we can eliminate two instructions by loading ES and DI directly as shown in Listing 22.2. **LISTING 22.2 L22-2.ASM** ```nasm ClearS proc near push bp ;save caller's BP mov bp,sp ;point to stack frame cmp word ptr [bp].BufSeg,0 ;skip the fill if a null jne Start ; pointer is passed cmp word ptr [bp].BufOfs,0 je Bye Start: cld ;make STOSW count up mov ax,[bp].Attrib ;load AX with attribute parameter and ax,0ff00h ;prepare for merging with fill char mov bx,[bp].Filler ;load BX with fill char and bx,0ffh ;prepare for merging with attribute or ax,bx ;combine attribute and fill char mov di,[bp].BufOfs ;load DI with target buffer offset mov es,[bp].BufSeg ;load ES with target buffer segment mov cx,[bp].BufSize ;load CX with buffer size rep stosw ;fill the buffer Bye: pop bp ;restore caller's BP ret EndMrk-RetAddr-2 ;return, clearing the parms from the stack ClearS endp ``` (The `OnStack` structure definition doesn't change in any of our examples, so I'm not going clutter up this chapter by reproducing it for each new version of `ClearS`.) Okay, loading ES and DI directly saves another four bytes. We've squeezed a total of 6 bytes—about 11 percent—out of `ClearS`. What next? Well, `LES` would serve better than two `MOV` instructions for loading ES and DI as shown in Listing 22.3. **LISTING 22.3 L22-3.ASM** ```nasm ClearS proc near push bp ;save caller's BP mov bp,sp ;point to stack frame cmp word ptr [bp].BufSeg,0 ;skip the fill if a null jne Start ; pointer is passed cmp word ptr [bp].BufOfs,0 je Bye Start: cld ;make STOSW count up mov ax,[bp].Attrib ;load AX with attribute parameter and ax,0ff00h ;prepare for merging with fill char mov bx,[bp].Filler ;load BX with fill char and bx,0ffh ;prepare for merging with attribute or ax,bx ;combine attribute and fill char les di,dword ptr [bp].BufOfs ;load ES:DI with target buffer ;segment:offset mov cx,[bp].BufSize ;load CX with buffer size rep stosw ;fill the buffer Bye: pop bp ;restore caller's BP ret EndMrk-RetAddr-2 ;return, clearing the parms from the stack ClearS endp ``` That's good for another three bytes. We're down to 43 bytes, and counting. We can save 3 more bytes by clearing the low and high bytes of AX and BX, respectively, by using `SUB *reg8,reg8*` rather than ANDing 16-bit values as shown in Listing 22.4. **LISTING 22.4 L22-4.ASM** ClearS proc near push bp ;save caller's BP mov bp,sp ;point to stack frame cmp word ptr [bp].BufSeg,0 ;skip the fill if a null jne Start ; pointer is passed cmp word ptr [bp].BufOfs,0 je Bye Start: cld ;make STOSW count up mov ax,[bp].Attrib ;load AX with attribute parameter sub al,al ;prepare for merging with fill char mov bx,[bp].Filler ;load BX with fill char sub bh,bh ;prepare for merging with attribute or ax,bx ;combine attribute and fill char les di,dword ptr [bp].BufOfs ;load ES:DI with target buffer ;segment:offset mov cx,[bp].BufSize ;load CX with buffer size rep stosw ;fill the buffer Bye: pop bp ;restore caller's BP ret EndMrk-RetAddr-2 ;return, clearing the parms from the stack ClearS endp Now we're down to 40 bytes—more than 20 percent smaller than the original code. That's pretty much it for simple instruction optimizations. Now let's look for instruction optimizations. It seems strange to load a word value into AX and then throw away AL. Likewise, it seems strange to load a word value into BX and then throw away BH. However, those steps are necessary because the two modified word values are ORed into a single character/attribute word value that is then used to fill the target buffer. Let's step back and see what this code really *does*, though. All it does in the end is load one byte addressed relative to BP into AH and another byte addressed relative to BP into AL. Heck, we can just do that directly! Presto—we've saved another 6 bytes, and turned two word-sized memory accesses into byte-sized memory accesses as well. Listing 22.5 shows the new code. **LISTING 22.5 L22-5.ASM** ```nasm ClearS proc near push bp ;save caller's BP mov bp,sp ;point to stack frame cmp word ptr [bp].BufSeg,0 ;skip the fill if a null jne Start ; pointer is passed cmp word ptr [bp].BufOfs,0 je Bye Start: cld ;make STOSW count up mov ah,byte ptr [bp].Attrib[1];load AH with attribute mov al,byte ptr [bp].Filler ;load AL with fill char les di,dword ptr [bp].BufOfs ;load ES:DI with target buffer segment:offset mov cx,[bp].BufSize ;load CX with buffer size rep stosw ;fill the buffer Bye: pop bp ;restore caller's BP ret EndMrk-RetAddr-2 ;return, clearing the parms from the stack ClearS endp ``` (We could get rid of yet another instruction by having the calling code pack both the attribute and the fill value into the same word, but that's not part of the specification for this particular routine.) Another nifty instruction-rearrangement trick saves 6 more bytes. `ClearS` checks to see whether the far pointer is null (zero) at the start of the routine...then loads and uses that same far pointer later on. Let's get that pointer into registers and keep it there; that way we can check to see whether it's null with a single comparison, and can use it later without having to reload it from memory. This technique is shown in Listing 22.6. **LISTING 22.6 L22-6.ASM** ```nasm ClearS proc near push bp ;save caller's BP mov bp,sp ;point to stack frame les di,dword ptr [bp].BufOfs ;load ES:DI with target buffer;segment:offset mov ax,es ;put segment where we can test it or ax,di ;is it a null pointer? je Bye ;yes, so we're done Start: cld ;make STOSW count up mov ah,byte ptr [bp].Attrib[1];load AH with attribute mov al,byte ptr [bp].Filler ;load AL with fill char mov cx,[bp].BufSize ;load CX with buffer size rep stosw ;fill the buffer Bye: pop bp ;restore caller's BP ret EndMrk-RetAddr-2 ;return, clearing the parms from the stack ClearS endp ``` Well. Now we're down to 28 bytes, having reduced the size of this subroutine by nearly 50 percent. Only 13 instructions remain. Realistically, how much smaller can we make this code? About one-third smaller yet, as it turns out—but in order to do that, we must stretch our minds and use the 8088's instructions in unusual ways. Let me ask you this: What do most of the instructions in the current version of `ClearS` do? They either load parameters from the stack frame or set up the registers so that the parameters can be accessed. Mind you, there's nothing wrong with the stack-frame-oriented instructions used in `ClearS`; those instructions access the stack frame in a highly efficient way, exactly as the designers of the 8088 intended, and just as the code generated by a high-level language would. That means that we aren't going to be able to improve the code if we don't bend the rules a bit. Let's think...the parameters are sitting on the stack, and most of our instruction bytes are being used to read bytes off the stack with BP-based addressing...we need a more efficient way to address the stack...*the stack*...THE STACK! Ye gods! That's easy—we can use the *stack pointer* to address the stack rather than BP. While it's true that the stack pointer can't be used for *mod-reg-rm* addressing, as BP can, it *can* be used to pop data off the stack—and `POP` is a one-byte instruction. Instructions don't get any shorter than that. There is one detail to be taken care of before we can put our plan into action: The return address—the address of the calling code—is on top of the stack, so the parameters we want can't be reached with `POP`. That's easily solved, however—we'll just pop the return address into an unused register, then branch through that register when we're done, as we learned to do in Chapter 14. As we pop the parameters, we'll also be removing them from the stack, thereby neatly avoiding the need to discard them when it's time to return. With that problem dealt with, Listing 22.7 shows the Zenned version of `ClearS`. **LISTING 22.7 L22-7.ASM** ```nasm ClearS procnear pop dx ;get the return address pop ax ;put fill char into AL pop bx ;get the attribute mov ah,bh ;put attribute into AH pop cx ;get the buffer size pop di ;get the offset of the buffer origin pop es ;get the segment of the buffer origin mov bx,es ;put the segment where we can test it or bx,di ;null pointer? je Bye ;yes, so we're done cld ;make STOSW count up rep stosw ;do the string store Bye: jmp dx ;return to the calling code ClearS endp ``` At long last, we're down to the bare metal. This version of `ClearS` is just 19 bytes long. That's just 37 percent as long as the original version, *without any change whatsoever in the functionality that `ClearS` makes available to the calling code*. The code is bound to run a bit faster too, given that there are far fewer instruction bytes and fewer memory accesses. All in all, the Zenned version of `ClearS` is a vast improvement over the original. Probably not the best possible implementation—*never say never!*—but an awfully good one.