7.6.2 MAT READ Statement
  The MAT READ statement assigns values from DATA
  statements to array elements. Subscripts define either the di-
  mensions of the array being created or the new dimensions
  of an existing array; subscripts are optional in MAT READ
  statements.

  If you do not provide enough data in DATA statements to fill
  the specified array, BASIC leaves the remaining array el-
  ements unchanged. If you provide more data values than
  there are array elements, BASIC assigns enough values to fill
  the array and leaves the DATA pointer at the next value.

  In the following example, BASIC fills matrix B with the first
  four DATA items, fills matrix C with the next four DATA
  values, and leaves the DATA pointer at the ninth value in the
  DATA list:
  EXAMPLE: Click to display example.

  Output
  MATRIX B
    1 2
    3 4
  MATRIX C
    5 6
    7 8