133 lines
4.5 KiB
ArmAsm
133 lines
4.5 KiB
ArmAsm
; Template and utility function for a single line of the GTE blitter. See the other GTE.Line.s file
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; for details on the general structure of this template.
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;
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; This is a variant that places the snippets inline within the code field. We give up
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; the speed of three-byte code sequences, but eliminate the double JMP and simplify the
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; handling of odd-alignment.
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;
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; This mode is best when the scenes are always complicated.
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;
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; Odd:
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MX %00
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entry_1 ldx #0000 ; patch with the address of the direct page tiles. Fixed.
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entry_2 ldy #0000 ; patch with the address of the line in the second layer. Set when BG1 scroll position changes.
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entry_3 lda #0000 ; patch with the address of the right edge of the line. Set when origin position changes.
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tcs
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entry_jmp jmp $2000 ; always jump into the same location. For odd line, the end
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; ; of the snippet will be patched to handle the right-edge case.
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right_odd sep #$20 ; enter here from the code field
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pha
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rep #$20
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jmp $2000 ; jump back into the code field
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jmp odd_exit
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jmp even_exit
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; Code field, each block is N bytes
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loop
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lda #1234 ; PEA $0000 becomes LDA #0000 / BRA / PHA
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bra l0
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jmp exit ; 'normal' exit point
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jmp left_odd ; handler for pushing a single byte to the left edge
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jmp right_odd ; handler for pushing a single byte to the right edge
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l0 pha ; always end with a PHA, this is the patch point
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lda (00),y ;
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and #MASK
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ora #data
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bra l1
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jmp exit ; 'normal' exit point
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jmp left_odd ; handler for pushing a single byte to the left edge
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jmp right_odd ; handler for pushing a single byte to the right edge
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l0 pha ; always end with a PHA, this is the patch point
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...
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jmp loop
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jmp even_exit
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left_odd sep #$20
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xba
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pha
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rep #$20
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exit jmp $0000 ; Jump to the next line. We set up the blitter to do 8 or 16 lines at a time
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; ; before restoring the machine state and re-enabling interrupts. This makes
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; ; the blitter interrupt friendly to allow things like music player to continue
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; ; to function.
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;
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; ; When it's time to exit, the next_entry address points to an alternate exit point
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; These are the special code snippets -- there is a 1:1 relationship between each snippet space
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; and a 3-byte entry in the code field. Thus, each snippet has a hard-coded JMP to return to
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; the next code field location
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;
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; The snippet is required to handle the odd-alignment in-line; there is no facility for
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; patching or intercepting these values due to their complexity. The only requirements
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; are:
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;
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; 1. Carry Clear -> 16-bit write and return to the next code field operand
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; 2. Carry Set
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; a. Overflow set -> Low 8-bit write and return to the next code field operand
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; b. Overflow clear -> High 8-bit write and exit the line
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; c. Always clear the Carry flags. It's actually OK to leave the overflow bit in
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; its passed state, because having the carry bit clear prevent evaluation of
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; the V bit.
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;
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; Snippet Samples:
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;
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; Standard Two-level Mix (27 bytes)
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;
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; Optimal = 18 cycles (LDA/AND/ORA/PHA)
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; 16-bit write = 23 cycles
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; 8-bit low = 35 cycles
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; 8-bit high = 36 cycles
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;
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; start lda (00),y
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; and #MASK
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; ora #DATA ; 14 cycles to load the data
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; bcs 8_bit
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; pha
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; out jmp next ; Fast-path completes in 9 additional cycles
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; 8_bit sep #$30 ; Switch to 8 bit mode
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; bvs r_edge ; Need to switch if doing the left edge
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; xba
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; r_edge pha ; push the value
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; rep #$31 ; put back into 16-bit mode and clear the carry bit, as required
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; bvs out ; jmp out and continue if this is the right edge
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; jmp even_exit ; exit the line otherwise
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; ;
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; ; The slow paths have 21 and 22 cycles for the right and left
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; ; odd-aligned cases respectively.
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snippets ds 32*82
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