To:	djgpp AT delorie DOT com
Date:	Tue, 5 Nov 1996 22:15:25 PST
Subject:	Help with At&t assembly
Message-ID:	<19961105.221529.2870.0.MSnakes@juno.com>
From:	msnakes AT juno DOT com (Matt J Reiferson)

Hi, 
	I am new to DJGPP and it's At&t assembly.  I am writing a ray caster.  Whoever is reading this, if you know about ray casting, you need to
write sliver routines.  I want this sliver routine to be in pure 32-bit
assembly.  The problem is I wrote the original engine with real mode 16-bit instructions on Intel syntax.  I figured out how to access the video
buffer using near pointers in C and how to create a double_buffer.  But
how do you access them in at&t assembly.  I want to be able to assign one pixel in the double_buffer a pixel in a texture from a pcx file. 
Having the textures loaded into memory already how do I do this.  Here
is the real-mode 32-bit asm instructions (Intel syntax).  By the way, this is also an external function.  Can anyone convert this function to
in-line DJGPP v2.0 At&t instructions.  One more thing, the code has
comments, they are not mine, this code was in the book Tricks of the
Game Programming Gurus, in the book in order to make the functoin faster
he converted it to asm.  Can you send all responses to MSnakes AT juno DOT com
it would be greatly appreciated.

	
;/////////////////////////////////////////////////////////////////////////////
;
;This function is an assembly language version of the C function that
renders
;a single textured sliver into the double buffer.  It uses the
pre-computed
;look up table for the scale indices to speed things up, moreover it uses
;global variables instead of variables passed on the stack.  I highly
suggest
;you understand the the C version first before trying this one!
;
;////////////////////////////////////////////////////////////////////////////

.MODEL MEDIUM,C             ; use medium memory model and C function
names

.CODE                       ; begin code segment

EXTRN double_buffer:DWORD   ; the external double buffer
EXTRN sliver_texture:DWORD  ; a pointer to the texture memory
EXTRN sliver_column:WORD    ; the current texture column
EXTRN sliver_top:WORD       ; the starting Y of the sliver
EXTRN sliver_scale:WORD     ; the over all height of the sliver
EXTRN sliver_ray:WORD       ; the current video column
EXTRN sliver_clip:WORD      ; how much of the texture is being clipped
EXTRN scale_row:WORD        ; the pointer to the proper row of
pre-computed scale
                            ; indices


PUBLIC Render_Sliver_32     ; export the function to the linker

Render_Sliver_32 PROC FAR C ; this is a C function and is far

.386                             ; use 80386 instructions, we need them for the
                                 ; extra segments fs,gs

        push si                  ; save registers we will obliterate
        push di

        les di, double_buffer    ; point es:di to double buffer

        mov dx,sliver_column     ; hold the column in dx

        lfs si, sliver_texture   ; fs:si points to texture memory

        ; offset = (sprite->y << 8) + (sprite->y << 6) + sprite->x

        mov bx,sliver_top        ; multiply Y by 320 to get proper offset
        shl bx,8
        mov ax,bx
        shr bx,2
        add bx,ax
        add bx,sliver_ray        ; add X
        add di,bx


        mov bx,sliver_clip       ; move important constants into registers
        mov ax,sliver_scale
        add ax,bx

Sliver_Loop:                     ; main loop


        ; double_buffer[offset] = work_sprite[work_offset+column]

        xchg dx,bx               ; exchange dx and bx since only bx can
be used
                                 ; as a index
        mov cl, BYTE PTR fs:[esi+bx] ; get texture pixel

        mov es:[di], cl             ; move it to screen

        xchg dx,bx                  ; restore dx and bx

        mov cx,bx                   ; get ready to access proper scale index

        ; row = scale_table[scale]

        mov dx, scale_row

        shl bx,1

        add bx,dx

        mov dx, WORD PTR [bx]       ; get scale index out of array

        add dx,sliver_column

        mov bx,cx

        ; offset      += SCREEN_WIDTH;

        add di,320                  ; move down to next video line

        inc bx                      ; increment counter
        cmp bx, ax
        jne Sliver_Loop             ; are we done?

        pop di                      ; restore registers
        pop si

        ret                         ; blaze

Render_Sliver_32 ENDP

END

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