BBC Micro Elite

Maths (Geometry): TAS3 (6502SP version)

```       Name: TAS3                                              [View in context]
Type: Subroutine
Category: Maths (Geometry)
Summary: Calculate the dot product of XX15 and an orientation vector

Calculate the dot product of the vector in XX15 and one of the orientation
vectors, as determined by the value of Y. If vect is the orientation vector,
we calculate this:

(A X) = vect . XX15
= vect_x * XX15 + vect_y * XX15+1 + vect_z * XX15+2

Arguments:

Y                    The orientation vector:

* If Y = 10, calculate nosev . XX15

* If Y = 16, calculate roofv . XX15

* If Y = 22, calculate sidev . XX15

Returns:

(A X)                The result of the dot product

.TAS3

LDX INWK,Y             \ Set Q = the Y-th byte of INWK, i.e. vect_x
STX Q

LDA XX15               \ Set A = XX15

JSR MULT12             \ Set (S R) = Q * A
\           = vect_x * XX15

LDX INWK+2,Y           \ Set Q = the Y+2-th byte of INWK, i.e. vect_y
STX Q

LDA XX15+1             \ Set A = XX15+1

JSR MAD                \ Set (A X) = Q * A + (S R)
\           = vect_y * XX15+1 + vect_x * XX15

STA S                  \ Set (S R) = (A X)
STX R

LDX INWK+4,Y           \ Set Q = the Y+2-th byte of INWK, i.e. vect_z
STX Q

LDA XX15+2             \ Set A = XX15+2

\ Fall through into MAD to set:
\
\   (A X) = Q * A + (S R)
\           = vect_z * XX15+2 + vect_y * XX15+1 +
\             vect_x * XX15
```