ISZ

Name

Increment index register skip if zero

Function

The content of the designated index register is incremented by 1.
The accumulator and carry/link are unaffected. If the result is zero,
the next instruction after ISZ is executed.
If the result is different from 0, program control is transferred
to the instruction located at the 8 bit address A2A2A2A2, A1A1A1A1 on
the same page (ROM) where the ISZ instruction is located.

Syntax

ISZ(R, 8-bit address)

Assembled

Binary

0111R A2A2A2A2, A1A1A1A1

Decimal

112, then incrementing by 1 until 127 (1st word)

Hexadecimal

0x70, then incrementing by 1 until 0x7F (1st word)

Symbolic

 ../../_images/isz-sym.png

Execution

2 words, 8-bit code and an execution time of 10.8 \({\mu}\) sec

Side-effects

Not Applicable

Implemented

isz

Detailed Description

The index register specified by REG is incremented by one. If the result is 0000, program execution continues with the next sequential instruction. If the result does not equal 0000 the 8 bits specified by ADDR replace the lowest 8 bits of the program counter. The highest 4 bits of the program counter are unchanged. Therefore, program execution continues at the specified address on the same page of memory in which the ISZ instruction is located.

The carry bit is not affected

../../_images/isz.png

NOTE: If ISZ is located on words 254 and 255 of a ROM page, when ISZ
is executed and the result is not zero, program control is transferred
to the 8-bit address located on the next page in sequence and not
on the same page where ISZ is located. .. rubric:: Example program

/ Example program
        org    ram
        fim    0p
lp      xch    2
        isz    0   lp
        end

The FIM instruction loads registers 0 and 1 with O. The XCH is then executed. Program execution continues until the ISZ is reached. Register 0 is incremented to contain 1, and since this result is non-zero, program control is transferred back to location labelled “lp”. This process continues until register 0 = 1111. Then the ISZ increments register 0 producing a result of OOOO, and execution continues with the instruction at after the ISZ (which is the END).