HP 71B FORTH/Assembler ROM

[robve]

(09-03-2022 01:40 AM)Garth Wilson Wrote:  

Quote:Regarding Forth on the 71B, as you are exploring and learning it, you will encounter Memory Lost far more often

Well, it has been a long time since I started into Forth, which was on the 71; but I already was familiar with stacks from the 41 and microprocessor work, and I don't recall ever having a crash in Forth on the 71. I did in BASIC with a particular bug in TRANSFORM though, and it was so bad I couldn't even bring it back with INIT:3.

Duly noted.

The HP-71B FORTH/Assembler ROM manual has lots of warnings (emphasis is mine) about memory lost and system reset issues, which aren't reassuring:

FORTH is similar to assembly language in its lack of user protection. In most cases FORTH will attempt to perform the specified operation, even if the operation will cause a Memory Lost condition. For instance, it is easy to write a FORTH loop that pushes a value onto the data stack 1,000,000 times. Execution of this loop will overwrite the user dictionary, the FORTH system variables, and the BASIC O/S variables. Eventually the machine will be too confused to continue and will perform a cold start. In other cases you might need to perform an INIT 3 to recover normal HP-71 operation.

Because of the complete access to the HP-71 memory space provided by FORTH, it is quite possible for a FORTH program to store inappropriate data into HP-71 operating system RAM. In many cases, this will cause a memory lost condition. Following a memory loss, the HP-71 will return to the BASIC environment.

Subroutine Return Stack. Return addresses are stored on an eight-level LIFO hardware stack. Subroutine call and return instructions automatically push and pop addresses on this stack. If a ninth address is pushed onto the stack, the oldest address will be lost and will be replaced by zero when it is eventually popped from the stack. Because the memory-reset code of the operating system resides at address 00000, excessive nesting of subroutine calls will cause a memory reset.

Anyway, still a great machine and ROM to learn Forth

- Rob