Chapter 14 Instruction logic of the MIDAC 211
2 It may contain a counter or tally which can be increased by a base instruction. This instruction makes use of the address modification digits to change the counter so as to count the number of traversals of a particular cycle of instructions.
Instruction types
Instructions used in MIDAC can be divided into three categories:
change of information, change of control, and transfer of information. The first category can be further subdivided into arithmetic and logical instructions. In the arithmetic instructions are included addition, subtraction, division, various forms of multiplication; power extraction, number shifting; and number conversion instructions. The sole logical instruction is extract, which modifies information in a nonarithmetic fashion.
The transfer of information or data transfer instructions include transfers of individual words or blocks of words into and out of the acoustic storage and drum and magnetic tape control.
The possible change of control instructions includes two comparisons that provide different future sequences dependent on the differences of two numbers. In the compare numbers or algebraic comparison, the difference is an algebraic, signed one. In the compare magnitudes or absolute comparison, the difference is one between absolute values. Two other instructions, file and base, perform other tasks beside transferring control. The file instruction transfers control unconditionally. The file instruction files or stores the contents of the base or instruction counter in a specific address position of a particular word in the storage. The base or tally instruction provides a method for referring addresses automatically relative to the address given by the base counter, irrespective of its contents. The base instruction also gives a conditional transfer of control.
The nineteen MIDAC instructions can be described function ally as follows:
Change of information
1 Add. (a ') + (b ') is placed in g '. Result must be less than 1 in absolute value.
2 Subtract. (a ') - (b ') is placed in g '. Result must be less than 1 in absolute value.
3 Multiply, Low Order. The least significant 44 binary digits of (a ') X (b ') are placed in g '.
4 Multiply, High Order. The most significant 44 binary digits of (a ') X (b ') are placed in g '.
5 Multiply, Rounded. The most significant 44 binary digits of (a ') X (b ') + 1 . 2-45 are placed in g '. The 1 . 2-45 is added if (a ') x (b ') is positive, and subtracted if (a ') X (b ') is negative.
6 Divide. The most significant 44 binary digits of (b ')/(a ') are placed in g '. (Note the inversion of order of a and b .) Result must be less than 1 in absolute value.
7 Power Extract. The number n . 2-44 is placed in g ' where n is the number of binary 0's to the left of the most significant binary 1 in (a '). The b digit is ignored; b may be any even number. If (a ') is all zeros, zero is placed in g '.
8 Shift Number. The 44 binary digits immediately to the right of the radix point in (a ') . 2(b ').244 are placed in g '. The result, in g ', is the equivalent of shifting (a ') n places, where n . 2-44 = (b ') and n positive indicates a shift left, n negative a shift right. If ÷ n÷ ³ 44, zero is placed in g '.
9 Extract or Logical Transfer. Those binary digits in (g '), including the sign digit, whose positions correspond to 1's in (b ') are replaced by the digits in the corresponding positions of (a ').
10 Decimal to Binary Conversion. This operation may be interpreted in two ways: (a) (a ') is considered as a binary- coded-decimal integer times 2-44. It is converted to the equivalent binary integer times 2-37 and the result is placed in g ', or (b) (a ') is considered as a binary-coded- decimal fraction, D. It is converted into an intermediate binary fraction, Bi, such that Bi = D x 1011 x 2-37 and the result placed in y'. To obtain B, the true binary equivalent of D, Bi must be multiplied by (10-11 x 237). However, since this factor is greater than 1 and therefore cannot be represented in the machine, two operations must be performed. For example,
Bi x (10-11 x 237 -1) = Bj
B = Bi + Bj
Here the b digit is ignored, and b may be any even number.
11 Binary-to-Decimal Conversion. (a '), considered as a binary fraction, is converted into the equivalent eleven-digit binary-coded-decimal fraction. The result is placed in g '. The b digit is ignored, and b may be any odd number.
Change of control
12 Compare Numbers. g can be relative only to the instruction counter. If (a ') ³ (b '), the contents of the instruction counter are increased by one as is normally done at the end of each instruction. If (a ') < (b '), the contents of the instruction counter are set to g '.