push - BASIC Commands

Name:

push

Syntax:

PUSH data { , data, data ... }

data - the byte data value, variable or constant, to be added to the stack.

Description:

Copy a word data value to the stack. The data may either be a numeric constant or the value held within a variable. Data can be bit, byte or word sized.

All existing stack values are moved up one place and the variable value is added to the bottom of the stack. The stack is 16 words in size and can therefore hold up to 16 word variables. It is separate to the stack used by pushram and popram commands. After 16 consequative pushes (without any pop) the earliest data is simply lost.

Pushing and popping order

Note that variables need to be popped in the reverse order to which they are pushed on to the stack. If b1 is pushed before b2, then b2 needs to be popped before b1.

When b1 and b2 are pushed in a single command, the order of those variables need to be reversed in the corresponding pop command -

    push b1, b2
    gosub myroutine
    pop b2, b1

This is the same as -

    push b1
    push b2
    gosub myroutine
    pop b2
    pop b1

Applies To:

20X2, 28X2, 40X2

See Also:

Related Create:

Simple push and pop

This example shows how a variable (b0 here) can be pushed to the stack, have its value changed, then have its original value restored after a pop.

Code Example:

main:	b0 = 0
	sertxd( "Before push, b0=", #b0, cr, lf )
	push b0
	sertxd( "After push, b0=", #b0, cr, lf )
	b0 = 99
	sertxd( "After change, b0=", #b0, cr, lf )
	pop b0
	sertxd( "After pop, b0=", #b0, cr, lf )
	end

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Pushing and popping multiple variables

Note that variables must be popped in the reverse order to which they are pushed on to the stack. Here b1 is pushed before b2, therefore b2 needs to be popped before b1.

Code Example:

main:	b1 = 11
	b2 = 22
	push b1
	push b2
	b1 = 99
	b2 = 99
	pop b2
	pop b1
	sertxd( "b1=", #b1, " b2=", #b2, cr, lf )
	end

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Pushing and popping multiple variables in same command

Note that variables must be popped in the reverse order to which they are pushed on to the stack. Here b1 is pushed before b2, therefore b2 needs to be popped before b1. When b1 and b2 are pushed in a single command, the order of those variables need to be reversed in the corresponding pop command.

Code Example:

main:	b1 = 11
	b2 = 22
	push b1, b2
	b1 = 99
	b2 = 99
	pop b2, b1
	sertxd( "b1=", #b1, " b2=", #b2, cr, lf )
	end

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Passing parameters using the stack

This examples demonstrates how parameters for a subroutine may be pushed on to the stack and then pulled from the stack within the subroutine whichcan perform actions depending on those values retrieved. Note that variables must be popped in the reverse order to which they are pushed.

Code Example:

main:	b0 = 11
	push 1, b0
	gosub mysub

	b1 = 22
	push 2, b1
	gosub mysub

	end
	
mysub:	pop b2
	pop b1	
	sertxd( "b1=", #b1, " b2=", #b2, cr, lf )
	return

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