Intel mcs-48 manual Subroutines

INSTRUCTION SET

2K words of program memory. Jumps to the second 2K of memory (4K words are directly addressible) are made by first executing a select memory bank instruction then execu- ting the jump instruction. The 2K boundary can only be crossed via a jump or subroutine call instruction i.e. the bank switch does not occur until a jump is executed. Once a memory bank has been selected all subse- quent jumps will be to the selected bank until another select memory bank instruction is executed. A subroutine in the opposite bank can be accessed by a select memory bank instruction followed by a call instruction. Upon completion of the subroutine execu- tion will automatically return to the original bank; however, unless the original bank is reselected, the next jump instruction en- countered will again transfer execution to the opposite bank.

Conditional jumps can test the following inputs and machine status:

TO Input pin

T1 Input pin

INT Input pin

Accumulator Zero

Any bit of Accumulator

Carry Flag

FO Flag

F1 Flag

Conditional jumps allow a branch to any address within the current page (256 words) of execution. The conditions tested are the instantaneous values at the time the condi- tional jump is executed. For instance, the jump on accumulator zero instruction tests the accumulator itself not an intermediate zero flag.

The decrement register and jump if not zero instruction combines a decrement and a branch instruction to create an instruction very useful in implementing a loop counter. This instruction can designate anyone of the 8 working registers as a counter and can effect a branch to any address within the current page of execution.

A single byte indirect jump instruction allows the program to be vectored to anyone of

several different locations based on the contents of the accumulator. The contents of the accumulator points to a location in program memory which contains the jump address. The 8-bit jump address refers to the current page of execution. This instruction could be used, for instance, to vector to any one of several routines based on an ASCII character which has been loaded in the accumulator. In this way ASCII key inputs can be used to initiate various routines.

Subroutines

Subroutines are entered by executing a call instruction. Calls can be made like uncondi- tional jumps to any address in a 2K word bank and jumps across the 2K boundary are executed in the same manner. Two separate return instructions determine whether or not status (upper 4-bits of PSW) is restored upon return from the subroutine.

The return and restore status instruction also signals the end of an interrupt service routine if one has been in progress.

Timer Instructions

The 8-bit on board timer/counter can be loaded or read via the accumulator while the counter is stopped or while counting. The counter can be started as a timer with an internal clock source or as an event counter or timer with an external clock applied to the T1 input pin. The instruction executed determines which clock source is used. A single instruction stops the counter whether it is operating with an internal or an external clock source. In addition, two instructions allow the timer interrupt to be enabled or disabled.

Control Instructions

Two instructions allow the external interrupt source to be enabled or disabled. Interrupts are initially disabled and are automatically disabled while an interrupt service routine is in progress and re-enabled afterward.

There are four memory bank select instruc- tions, two to designate the active working register bank and two to control program

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