*1

((QAPLT1 for Algol library issue 6, 4/11/12)

(This Elliott SIR subroutine was written by Terry Froggatt,
 after refurbishing a brace of Benson-Lehner plotters in 2012.
 It is a direct replacement for the library tape version, but
 substantially smaller, and with several minor corrections)

[QAPLT1 QPLOT SETORI MOVEPE DRAWLI]

QAPLT1

BASE=100

PLOTAD=4864               (Standard Plotter Address)

(Entry points QPLOT+1 & QPLOT+2, and parameters X2,Y2,X1,Y1,
 are accessed as offsets of QPLOT, as part of the published
 user interface, and SENSE has been added in this version)

PENPOS    +0              (+0=Unknow, +16=Raised, +32=Lowered)
SENSE     +1              (+0=Benson-Lehner, +1=QPLOT)
QPLOT                     (Loaded into B-reg before main loop)
STEPS     >1              (Only required in main loop)
          8     LINE      (Draw Line from X1,Y1 to X2,Y2)
          8     ;+0       (Draw Character given 6-bit code)

X2        +0              (QPLOT+3)
Y2        +0              (QPLOT+4)
X1        +0              (QPLOT+5)
Y1        +0              (QPLOT+6)
X0                        (This must directly follow Y1, and is
                           used briefly before AXIS is required)

AXIS      >1              (Axis step has just one bit)
DIAG      >1              (Diagonal step has two bits)
LARGE     >1              (Larger of NS & EW Deltas)
SMALL     >1              (Smaller of NS & EW Deltas)
DELTA     >1              (Used for both Abs Delta NS and
                           Step Direction Discriminator)

LINE      0     QPLOT
          /4    1
          7     VERTOK
          6     +1
          7     LOWER
          4     +16
          8     RAISE
LOWER     4     PENPOS
          2     +32
          7     VERTOK
          4     +32
RAISE     5     PENPOS
          15    PLOTAD    (Output to Plotter)

VERTOK    0     SENSE     (+0=Benson-Lehner, +1=QPLOT)
          /4    X1        (4 X1 or 4 Y1)
          /2    X2        (2 X2 or 4 Y2)
          9     ;+4       (Test North or South)
          5     DELTA     (Abs Delta NS)
          4     +5        (Bits for North East step)
          8     ;+4       (North)
          2     +0        (South)
          5     DELTA     (Abs Delta NS)
          4     +9        (Bits for South East step)
          5     DIAG      (Provisionally Set Diagonal)

          4     X2
          5     X0
          /4    Y2        (4 Y2 or 4 X1)
          /2    Y1        (2 Y1 or 2 X0=X2)
          9     ;+2       (Test East or West)
          8     ;+3       (East)
          2     +0        (West)
          10    DIAG      (Change NE to NW or SE to SW)
          5     LARGE     (Provisionally Abs Delta EW)
          5     SMALL     (Provisionally Abs Delta EW)

          2     DELTA     (Abs Delta NS - Abs Delta EW)
          9     ;+5
          4     DELTA     (Abs Delta NS)
          5     LARGE     (Abs Delta NS >= Abs Delta EW)
          4     +12       (Bits for North or South step)
          8     ;+4
          4     DELTA     (Abs Delta NS)
          5     SMALL     (Abs Delta NS < Abs Delta EW)
          4     +3        (Bits for East or West step)
          6     DIAG      (Diagonal step has two bits)
          5     AXIS      (Axis step has just one bit)

          4     X2
          5     X1
          4     Y2
          5     Y1

          0     LARGE     (Interrupt cannot follow this so Q is safe)
          3     DELTA     (Set Mid-Point Discriminator to LARGE/2.
                           It would be more correct to set to LARGE,
                           then to subtract 2*SMALL & add 2*LARGE
                           within the loop, but this might differ
                           slightly from the previous version)

          4     LARGE
          0     QPLOT     (Frees up QPLOT for use as STEPS)
NXSTEP    /7    2         (Test at start of loop in case QPLOT has)
          5     STEPS     (been called just to raise or lower pen)

          4     SMALL
          2     DELTA     (Subtract SMALL, LARGE times)
          9     ;+4
          5     DELTA
          4     AXIS      (Take one step along axis)
          8     ;+4
          1     LARGE     (Add LARGE, SMALL times)
          5     DELTA
          4     DIAG      (Take one step along diagonal)
          15    PLOTAD    (Output to Plotter)

          4     STEPS
          1     -1
          8     NXSTEP

(Algol Interface)

SETORI    /14   2
          >1
          0     BASE+38
          /4    6
          2     +1
          7     ;+2       (If parameter = 1, Y-axis is across paper)
          4     +1        (All other values, X-axis is across paper)
          5     SENSE
          4     +16       (Raise)
          5     PENPOS
          15    PLOTAD
          4     -3400     (Maximum width of any plotter model)
          5     STEPS
          4     +2        (Left)
          15    PLOTAD
          10    STEPS
          4     STEPS
          9     ;-4
          5     X1
          5     Y1
          8     ;+4       (Avoid increment if origin is zero)
          4     +1        (Right)
          15    PLOTAD
          10    STEPS
          /4    3
          2     STEPS
          9     ;-5
          0     SETORI+1
          /8    1

MOVEPE    /14   2
          >1
          11    QPLOT
          8     GETXY2
          +1
          0     MOVEPE+1
          /8    1

DRAWLI    /14   2
          >1
          11    QPLOT
          8     GETXY2
          +2
          0     DRAWLI+1
          /8    1

GETXY2    0     BASE+38
          /4    3
          5     X2
          /4    6
          5     Y2
          8     QPLOT+1

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