startup.s breakdown for hello Jag users!

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startup.s breakdown for hello Jag users!

Post by a31chris » Sun Jun 21, 2015 5:01 pm

VladR has been nice enough to share his working through the startup.s file. This for those who don't know this is the file (or one like it with the same name) that bootstraps the Jaguar every time a program is run.

First I will start with posting the actual startup.s code from 'Hello Jag users!

Thanks to VladR for this. Reprinted with permission.

Code: Select all

	.include    ""

; Begin STARTUP PICTURE CONFIGURATION -- Edit this to change startup picture

PPP     	.equ    8      			; Pixels per Phrase (1-bit)
BMP_WIDTH   	.equ    320      		; Width in Pixels
BMP_HEIGHT  	.equ    200    			; Height in Pixels


; Globals
		.globl	gSetOLP
		.globl	olp2set
		.globl	ticks

		.globl  a_vdb
		.globl  a_vde
		.globl  a_hdb
		.globl  a_hde
		.globl  width
		.globl  height
                .globl  _vidmem
; Externals
		.extern	___main
                .extern _SetPallete

BMP_PHRASES 	.equ    (BMP_WIDTH/PPP) 	; Width in Phrases
BMP_LINES   	.equ    (BMP_HEIGHT*2)  	; Height in Half Scanlines
BITMAP_OFF  	.equ    (2*8)       		; Two Phrases
LISTSIZE    	.equ    5       		; List length (in phrases)

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;; Program Entry Point Follows...


		move.l  #$70007,G_END		; big-endian mode
		move.l  #$70007,D_END
		move.w  #$FFFF,VI       	; disable video interrupts

		move.l  #INITSTACK,a7   	; Setup a stack
		jsr 	InitVideo      		; Setup our video registers.
		jsr 	InitLister     		; Initialize Object Display List
		jsr 	InitVBint      		; Initialize our VBLANK routine

;;; Sneaky trick to cause display to popup at first VB

		move.l	#$0,listbuf+BITMAP_OFF
		move.l	#$C,listbuf+BITMAP_OFF+4

		move.l  d0,olp2set      	; D0 is swapped OLP from InitLister
		move.l  #gSetOLP,G_PC   	; Set GPU PC
		move.l  #RISCGO,G_CTRL  	; Go!
		move.l  G_CTRL,d0   		; Wait for write.
		andi.l  #$1,d0
		bne 	waitforset

		move.w  #$6C7,VMODE     	; Configure Video

	    jmp 	___main			; Jump to main code

; Procedure: gSetOLP
;            Use the GPU to set the OLP and quit.
;    Inputs: olp2set - Variable contains pre-swapped value to stuff OLP with.
; NOTE!!!: This code can run in DRAM only because it contains no JUMP's or
;          JR's. It will generate a warning with current versions of MADMAC
;          because it doesn't '.ORG'.
		movei   #olp2set,r0   		; Read value to write
		load    (r0),r1

		movei   #OLP,r0       		; Store it
		store   r1,(r0)

		moveq   #0,r0         		; Stop GPU
		movei   #G_CTRL,r1
		store   r0,(r1)
		nop             		; Two "feet" on the brake pedal


olp2set:    	.ds.l   1           		; GPU Code Parameter


; Procedure: InitVBint
; Install our vertical blank handler and enable interrupts

		move.l  d0,-(sp)

		move.l  #UpdateList,LEVEL0	; Install 68K LEVEL0 handler

		move.w  a_vde,d0        	; Must be ODD
		ori.w   #1,d0
		move.w  d0,VI

		move.w  #C_VIDENA,INT1         	; Enable video interrupts

		move.w  sr,d0
		and.w   #$F8FF,d0       	; Lower 68k IPL to allow
		move.w  d0,sr           	; interrupts

		move.l  (sp)+,d0
; Procedure: InitVideo (same as in vidinit.s)
;            Build values for hdb, hde, vdb, and vde and store them.
		movem.l d0-d6,-(sp)
		move.w  CONFIG,d0      		 ; Also is joystick register
		andi.w  #VIDTYPE,d0    		 ; 0 = PAL, 1 = NTSC
		beq 	palvals

		move.w  #NTSC_HMID,d2
		move.w  #NTSC_WIDTH,d0

		move.w  #NTSC_VMID,d6
		move.w  #NTSC_HEIGHT,d4

		bra 	calc_vals
		move.w  #PAL_HMID,d2
		move.w  #PAL_WIDTH,d0

		move.w  #PAL_VMID,d6
		move.w  #PAL_HEIGHT,d4

		move.w  d0,width
		move.w  d4,height

		move.w  d0,d1
		asr 	#1,d1         	 	; Width/2

		sub.w   d1,d2         	  	; Mid - Width/2
		add.w   #4,d2         	  	; (Mid - Width/2)+4

		sub.w   #1,d1         	  	; Width/2 - 1
		ori.w   #$400,d1      	  	; (Width/2 - 1)|$400
		move.w  d1,a_hde
		move.w  d1,HDE

		move.w  d2,a_hdb
		move.w  d2,HDB1
		move.w  d2,HDB2

		move.w  d6,d5
		sub.w   d4,d5
		move.w  d5,a_vdb

		add.w   d4,d6
		move.w  d6,a_vde
		move.w  a_vdb,VDB
		move.w  #$FFFF,VDE
		move.l  #0,BORD1        	; Black border
		move.w  #0,BG           	; Init line buffer to black
		movem.l (sp)+,d0-d6

; InitLister: Initialize Object List Processor List
;    Returns: Pre-word-swapped address of current object list in d0.l
;  Registers: d0.l/d1.l - Phrase being built
;             d2.l/d3.l - Link address overlays
;             d4.l      - Work register
;             a0.l      - Roving object list pointer
		movem.l d1-d4/a0,-(sp)		; Save registers
		lea     listbuf,a0
		move.l  a0,d2           	; Copy

		add.l   #(LISTSIZE-1)*8,d2  	; Address of STOP object
		move.l	d2,d3			; Copy for low half

		lsr.l	#8,d2			; Shift high half into place
		lsr.l	#3,d2
		swap	d3			; Place low half correctly
		clr.w	d3
		lsl.l	#5,d3

; Write first BRANCH object (branch if YPOS > a_vde )

		clr.l   d0
		move.l  #(BRANCHOBJ|O_BRLT),d1  ; $4000 = VC < YPOS
		or.l	d2,d0			; Do LINK overlay
		or.l	d3,d1
		move.w  a_vde,d4                ; for YPOS
		lsl.w   #3,d4                   ; Make it bits 13-3
		or.w    d4,d1

		move.l	d0,(a0)+
		move.l	d1,(a0)+

; Write second branch object (branch if YPOS < a_vdb)
; Note: LINK address is the same so preserve it

		andi.l  #$FF000007,d1           ; Mask off CC and YPOS
		ori.l   #O_BRGT,d1      	; $8000 = VC > YPOS
		move.w  a_vdb,d4                ; for YPOS
		lsl.w   #3,d4                   ; Make it bits 13-3
		or.w    d4,d1

		move.l	d0,(a0)+
		move.l	d1,(a0)+

; Write a standard BITMAP object
		move.l	d2,d0
		move.l	d3,d1

		ori.l  #BMP_HEIGHT<<14,d1       ; Height of image

		move.w  height,d4           	; Center bitmap vertically
		sub.w   #BMP_HEIGHT,d4
		add.w   a_vdb,d4
		andi.w  #$FFFE,d4               ; Must be even
		lsl.w   #3,d4
		or.w    d4,d1                   ; Stuff YPOS in low phrase

		move.l	#_vidmem,d4
		lsl.l	#8,d4
		or.l	d4,d0

		move.l	d0,(a0)+
		move.l	d1,(a0)+
		movem.l	d0-d1,bmpupdate

; Second Phrase of Bitmap
		move.l	#BMP_PHRASES>>4,d0	; Only part of top LONG is IWIDTH
		move.l  #O_DEPTH8|O_NOGAP,d1   ; Bit Depth = 16-bit, Contiguous data

		move.w  width,d4            	; Get width in clocks
		lsr.w   #2,d4               	; /4 Pixel Divisor
		sub.w   #BMP_WIDTH,d4
		lsr.w   #1,d4
		or.w    d4,d1


		move.l	d0,(a0)+
		move.l	d1,(a0)+

; Write a STOP object at end of list
		clr.l   (a0)+
		move.l  #(STOPOBJ|O_STOPINTS),(a0)+

; Now return swapped list pointer in D0

		move.l  #listbuf,d0
		swap    d0

		movem.l (sp)+,d1-d4/a0

; Procedure: UpdateList
;        Handle Video Interrupt and update object list fields
;        destroyed by the object processor.

		move.l  a0,-(sp)

		move.l  #listbuf+BITMAP_OFF,a0

		move.l  bmpupdate,(a0)      	; Phrase = d1.l/d0.l
		move.l  bmpupdate+4,4(a0)

		add.l	#1,ticks		; Increment ticks semaphore

		move.w  #$101,INT1      	; Signal we're done
		move.w  #$0,INT2

		move.l  (sp)+,a0




listbuf:    	.ds.l   LISTSIZE*2  		; Object List
bmpupdate:  	.ds.l   2       		; One Phrase of Bitmap for Refresh
ticks:		.ds.l	1			; Incrementing # of ticks
a_hdb:  	.ds.w   1
a_hde:      	.ds.w   1
a_vdb:      	.ds.w   1
a_vde:      	.ds.w   1
width:      	.ds.w   1
height:     	.ds.w   1
                .ds.b   64000

VladR wrote:I spent some time in Assembler, going over the initialization of the video mode and object processor in file startup.s.

I finally forced myself to crosscheck the docs against several of the registers used in initialization of the video mode, as I mentioned last week that I'd like to do some experiments with the CRY mode (soon-ish), which required breaking down some constants (hence postponing for later).

In the statup.s, in the section waitforoffset, there is this line:

Code: Select all

		move.w  #$6C7,VMODE     	; Configure Video: 
That is not helpful. At all.

VMODE is register F00028 (described in detail in docs page 12)

The value $6C7 (hex) is 0000 0110 1100 0111 (binary)
Checking against JAGUAR.H, the above value can be rewritten as:
Now that is infinitely mode readable as $6C7. Here's the description copied from the docs:
bit 0: VIDEN: Enable Video Interrupts
bit 6: CSYNC: Enables composite sync on the vertical sync output
bit 7: BGEN: Clears the line buffer to the colour in the background register after displaying the contents. This only has effect in CRY and RGB16 modes
bits 1-3: MODE = 3 RGB16: 16-bit RGB. Each 32-bit entry in the line buffer is treated as two 16-bit RGB pixels
bits 9-11: PWIDTH =3 PWIDTH4: This field determines the width of pixels in video clock cycles. The width is one more than the value in this field.
Of interest are the bits for MODE and PWIDTH. Checking JAGUAR.H reveals the following for MODE:

Code: Select all

#define CRY16		0x0000		/* 16-bit CRY mode */
#define RGB24		0x0002		/* 24-bit RGB mode */
#define DIRECT16	0x0004		/* 16-bit Direct mode */
#define RGB16		0x0006		/* 16-bit RGB mode */
Changing RGB16 to CRY16 should change the video mode to CRY. Changing it to RGB24 should change it to 24-bit.

I suspect the OP would have to be instructed to use O_DEPTH16 instead of O_DEPTH8 (and maybe a pitch would have to be adjusted too) in its second phrase - but I'll leave that exercise for later, when I'll play with CRY mode, as I will for sure have to reread the section on Object processor's bitmap definitions.

Now, let's get back to bits of PWIDTH:

Code: Select all

#define PWIDTH1	0x0000		/* Pixel Dividers */
#define PWIDTH2	0x0200
#define PWIDTH3	0x0400
#define PWIDTH4	0x0600
#define PWIDTH5	0x0800
#define PWIDTH6	0x0A00
#define PWIDTH7	0x0C00
#define PWIDTH8	0x0E00  
From docs:
PWIDTH: This field determines the width of pixels in video clock cycles. The width is one more than the value in this field.
So, this is where the actual horizontal resolution is being set. This must be sync'ed with the contents of the second phrase for OP:

Code: Select all

		move.w  width,d4            	; Get width in clocks
		lsr.w   #2,d4      	; /4 Pixel Divisor
Backtracking the ASM code few pages above reveals that width = NTSC_WIDTH, which according to JAGUAR.H is 1409.

Hence the code above can be rewritten as equation: d4 = NTSC_WIDTH / (2 ^ 2) = 1409 / 4 = 352

As vertical resolutions are only few and between (240p vs 480i), the default startup.s creates the resolution 352x240.

Experimenting with different values for the divisor should result in different horizontal resolutions. This could be a useful feature for VRBasic.
What came after the Jaguar was the PS1 which for all it's greatness, ushered in corporate development and with it the bleached, repetitive, bland titles which for the most part we're still playing today. - David Wightman

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