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Further reading □ Overview □ History □ Charactron Tube □ Fonts □ SC4020 Speed □ Architecture □ Tube □ 16mm Camera □ Hardcopy □ Vought Camera □ Order Code □ Printing □ Drawing □ Film Production □ UAIDE 1970 □ UAIDE Contents 1964-1973 □ UAIDE Papers □ 1968: Honey □ 1968: Sarno □ 1968: Yarbrough □ SC4020 Manuals □ Engineers Training □ Information Manual □ Programmers' Reference Manual □ Other □ Diagrams drawn by computer
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⟶ ACL ⟶ Technology ⟶ SC 4020
⟶ ACL ⟶ Technology ⟶ SC 4020
ACL ACD C&A INF CCD CISD Archives
Further reading ⇓

□ Overview
□ History
□ Charactron Tube
□ Fonts
□ SC4020 Speed
□ Architecture
□ Tube
□ 16mm Camera
□ Hardcopy
□ Vought Camera
□ Order Code
□ Printing
□ Drawing
□ Film Production
□ UAIDE 1970
□ UAIDE Contents 1964-1973
UAIDE Papers
□ 1968: Honey
□ 1968: Sarno
□ 1968: Yarbrough
SC4020 Manuals
□ Engineers Training
□ Information Manual
□ Programmers' Reference Manual
Other
□ Diagrams drawn by computer

SC4020 Instructions

SC4020 Drawing

The SC4020 order code of 36-bit word instructions in addition to allowing the device to act as a fast lineprinter also provided commands for plotting characters or drawing lines that allowed the SC4020 to also act as a graphical output device.

The PLOT command allowed one of the 64 characters in the CHARACTRON tube mask to be plotted at a specified point on the (0,0) to (1023,1023) display area.

The DRAW VECTOR command specified a starting position for the line to be drawn, the DX and DY displacement of the end position of the line from the start and two flags, SX and SY, that indicated whether the X and Y end points of the line were a positive or negative displacement from the start point.

The two displacements are defined by 6 bits each giving a displacement of 0 to 63 units from the start position in X and Y. In consequence, drawing a line across the complete drawing area required the line to be drawn in up to 16 increments. Most of the graphics systems in use with the SC4020 shielded the computation of these increments from the user. As far as the user was concerned, lines could be drawn from any X,Y position on the display space to any other. If the graphics system did a good job at specifying the increments, it was not possible to realise that multiple lines had been drawn to achieve the one vector.

A minor hardware problem was that the short vectors were of different length. The DRAW VECTOR command itself could generate lines that vary in length from 63 to about 88 pixels in length depending on their orientation and, if the graphics system did not equalise the lengths of increments, they could range from 1 to 88. In consequence, it was important that the SC4020 engineer ensured that the intensity of lines did not vary dependent on their length. Not always that easy with the hardware available at the time.

In most drawings, the size of the hardware character set was too small to be of much value apart from plotting points. In consequence, most systems in use provided their own software character fonts drawn using the vector commands.

The commands GENERATE X-AXIS and GENERATE Y-AXIS were rarely used apart from simulating graph paper.

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