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Moving: MVT3

[NES version, Bank 0]

Name: MVT3 [Show more] Type: Subroutine Category: Moving Summary: Calculate K(3 2 1) = (x_sign x_hi x_lo) + K(3 2 1)
Context: See this subroutine in context in the source code References: This subroutine is called as follows: * MAS1 calls MVT3 * MV40 calls MVT3 * TAS1 calls MVT3

Add an INWK position coordinate - i.e. x, y or z - to K(3 2 1), like this: K(3 2 1) = (x_sign x_hi x_lo) + K(3 2 1) The INWK coordinate to add to K(3 2 1) is specified by X.
Arguments: X The coordinate to add to K(3 2 1), as follows: * If X = 0, add (x_sign x_hi x_lo) * If X = 3, add (y_sign y_hi y_lo) * If X = 6, add (z_sign z_hi z_lo)
Returns: A Contains a copy of the high byte of the result, K+3 X X is preserved
.MVT3 LDA K+3 ; Set S = K+3 STA S AND #%10000000 ; Set T = sign bit of K(3 2 1) STA T EOR INWK+2,X ; If x_sign has a different sign to K(3 2 1), jump to BMI MV13 ; MV13 to process the addition as a subtraction LDA K+1 ; Set K(3 2 1) = K(3 2 1) + (x_sign x_hi x_lo) CLC ; starting with the low bytes ADC INWK,X STA K+1 LDA K+2 ; Then the middle bytes ADC INWK+1,X STA K+2 LDA K+3 ; And finally the high bytes ADC INWK+2,X AND #%01111111 ; Setting the sign bit of K+3 to T, the original sign ORA T ; of K(3 2 1) STA K+3 RTS ; Return from the subroutine .MV13 LDA S ; Set S = |K+3| (i.e. K+3 with the sign bit cleared) AND #%01111111 STA S LDA INWK,X ; Set K(3 2 1) = (x_sign x_hi x_lo) - K(3 2 1) SEC ; starting with the low bytes SBC K+1 STA K+1 LDA INWK+1,X ; Then the middle bytes SBC K+2 STA K+2 LDA INWK+2,X ; And finally the high bytes, doing A = |x_sign| - |K+3| AND #%01111111 ; and setting the C flag for testing below SBC S ORA #%10000000 ; Set the sign bit of K+3 to the opposite sign of T, EOR T ; i.e. the opposite sign to the original K(3 2 1) STA K+3 BCS MV14 ; If the C flag is set, i.e. |x_sign| >= |K+3|, then ; the sign of K(3 2 1). In this case, we want the ; result to have the same sign as the largest argument, ; which is (x_sign x_hi x_lo), which we know has the ; opposite sign to K(3 2 1), and that's what we just set ; the sign of K(3 2 1) to... so we can jump to MV14 to ; return from the subroutine LDA #1 ; We need to swap the sign of the result in K(3 2 1), SBC K+1 ; which we do by calculating 0 - K(3 2 1), which we can STA K+1 ; do with 1 - C - K(3 2 1), as we know the C flag is ; clear. We start with the low bytes LDA #0 ; Then the middle bytes SBC K+2 STA K+2 LDA #0 ; And finally the high bytes SBC K+3 AND #%01111111 ; Set the sign bit of K+3 to the same sign as T, ORA T ; i.e. the same sign as the original K(3 2 1), as STA K+3 ; that's the largest argument .MV14 RTS ; Return from the subroutine