A long time ago I had an apple ][ .
I remember the command call – 151 But I can not remember what it did ?
A long time ago I had an apple ][ .
I remember the command call – 151 But I can not remember what it did ?
Call -151 enters the monitor, 3D0G brings you back to BASIC, and typing a slot # in the monitor followed by Ctrl-P will boot that device. Amazing what one remembers after 20 years!
May I also add that -151 is apple ]['s way of expressing hex number which should mean $FF69 (hex syntax used in Apple II i.e. 0xFF68). The CALL is an Apple Basic command that invokes an assembly subroutine given by the argument (-151 here). IIRC, this command can accept an address as negative decimal value for addresses between $8000-$FFFF using 2's complement interpretation.
For those who are interested in history, here is the Apple ]['s monitor rom listing (in 6502 assembly) and address $FF69 is having the label MONZ which is the start of the command prompt that process machine code processing commands from user. One that uses a '*' as the prompt. A very primitive command prompt.
As a side note, the reason why this is a negative number and not the proper CALL 65385 is because the very first form of BASIC for the Apple II was known as Integer BASIC. It only understood signed 16-bit Integer values from -32768 to 32767, and so it is impossible to directly address memory beyond 32767 in the normal positive value manner.
If you tried actually typing POKE 49200,0 or CALL 65385 in Integer BASIC you will get a message like ">32767 ERR"
When the replacement Microsoft Applesoft BASIC (yes, from them) with floating point numbers was introduced, they included support for the negative POKE values for some degree of backwards compatibility for the older Integer BASIC programs. Though this compatibility is limited, as Applesoft lacks other programming features of Integer like the MOD division remainder.
Due to the strong influence of early Integer BASIC programming methods, there are many PEEK POKE and CALL commands that are generally only known by their hexadecimal and negative decimal values, but not by their positive decimal values.
CALL -151
Enter the machine code monitor -
http://www.skepticfiles.org/cowtext/apple/memorytx.htm
Update:
That link appears to be dead, here's a Wayback Machine alternative:
Here's the full article just in case Wayback goes away: