Programming instructions

01 1121314151617 ~8~9~10~11~12~13~14~15~16~17

vvt1

Operation Indirect

Operand Address

Code Address

(Defer)

Figure 5 - Memory reference instruction format

INDIRECT ADDRESSING

When indirect addressing is specified, the address part (bits 5-17) of a

memory reference instruction is interpreted as the address of a cell con-

taining not the operand, but the address of the operand. Consider the

instruction add A. Normally, A is interpreted as the address of the cell

containing the quantity be be added to the AC. Thus, if cell 100 contains

the number 576, the instruction

add 100

will cause the quantity 576 to be added to the AC. Now suppose that cell

576 contains the number 1135. The instruction

add i 100

(where i signifies indirect addressing) will cause the computer to take the

number 576, which is in cell 100, as the effective address of the instruction,

and the number in cell 576 as the operand. Hence this instruction will

result in the quantity 1135 being added to the AC.

If, when indirect addressing is indicated, the memory cell addressed by

the instruction is one of those in locations 10-17, the contents of that cell

are incremented by one and the result taken as the effective address.

This feature is called auto-indexing. If memory cell 12 contains the number

200, the instruction

add i 12

will cause the number in cell 200 + 1 to be added to the AC.

1’S COMPLEMENT ARITHMETIC

When two numbers are added together in l’s complement arithmletic

(see add instruction in following table), a 1 carried out of the high-order

position will be added to the low-order digit, as follows:

110101001100011

011001010111101

rl 001110100100000

00111010010000~