Your link to Retrocomputing is very inspirative, thank you. I'm glad to help! Re-reviewing my answer there, it turns out Ralf Brown also chose to zero the environment block field of the resident PSP: xor ax,ax xchg ax,es:[002Ch] ; get and zero environment segment mov es,ax mov ah,49h ; since we wil...

However, that means if only a very small UMB is available, you may fail to allocate it when it is between 144 and 288 Bytes in size, even though that would suffice to install the resident portion. I thought it necessary to tell DOS that PSP of the resident has changed (was moved to upper memory). T...

I am not sure now why I chose to allocate this much upper memory, perhaps my DOS 5 refused to terminate and left resident the incomplete PSP in those times (30 years ago). I don't think so. You are relocating only the PSP contents up to 53h with your relocated resident code after that point already...