HP-29C Neutral Operations

[teenix]

Hi all,

I was playing around with the 29C emulator and reading the owner's manual about Neutral operations which are not supposed to affect the stack lift status from a previous operation.

For example, pressing [Enter] disables the stack lift so the next number entry just overwrites the X register contents without lifting the stack. So, after pressing [Enter], if I key in a neutral operation like [f][Fix][3], the stack lift status should not change, so entering [3] should just overwrite the X register but is pushes the stack.

A key sequence might be:

[1] [Enter] (X=1, Y= 1, Z=0, T=0) Stack lift disabled

[f][FIX][3] (X=1, Y=1, Z=0, T=0) Supposedly still disabled

[4]

According to manual
(X=4, Y=1, Z-0, T=0)

In the emulator I get
(X=4, Y=1, Z=1, T=0)

This is the same issue for the 19C emulator, however the 25C works as the manual says.

Just wondering if this happens in a real 19C or 29C if anyone has the time.

At the HP microcode level for the 19C, when the STATUS register flag [9] is set, the stack lift is active. This flag does get set for [f][fix][n] with microcode instruction [1 -> s9] (Code $244) at ROM address $038. This same code appears at the same ROM address in the data I extracted from reading a 19C ROM chip (1818-0432).

cheers

Tony