SECTION ONE. OUR COMPUTER ABACUS
The abacus in this program consists of a rectangular frame with 15 columns of beads arranged horizontally. Each column is separated into two sections by a horizontal crosspiece with one bead over the crosspiece and four beads under it. By moving the beads towards the crosspiece or away from it one can perform arithmetic computations.
The four beads below the crosspiece each represent the number 1. The one bead above the crosspiece represents the number 5. The columns are numbered from 1 to 15 starting at the left side.
The small rectangles on the crosspiece at every third column indicate reference columns. They help align number s by places (unit, 1,000's 1,000,000's etc.)
The eighth column is designated as the unit digit column (or unit reference column) in most problems. If a problem uses too many digits to fit in the first eight columns, then the unit digit column can be moved three or six columns to the right. The following diagram shows the abacus as it appears in the program.