the orioginaal OSC pin ( pin 35 ) is coonnected.. its function is to use the 18K resistor on the zx81 as a pulldown. it could easily be left unconnected and a pull down placed on the board instead.
Big bang rom ?? its address 101 for the features bits, 1001 is used to activate clock doubling ( or maybe quadrupling) when in fast mode.
as for address 101 ( $65 in hex ) its a spare byte in the standard rom.
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L is loaded with the 'data byte'.
0056 ERROR-2 POP HL
LD L,(HL)
0058 ERROR-3 LD (ERR-NR),L
LD SP,(ERR-SP)
CALL 0207,SLOW/FAST
JP 14BC,SET-MEM
DEFB +FF
BIG BANG X2 pulled from zx81x2c.asm
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; --------------------
; THE 'ERROR-2' BRANCH
; --------------------
; This is a continuation of the error restart.
; If the error occurred in runtime then the error stack pointer will probably
; lead to an error report being printed unless it occurred during input.
; If the error occurred when checking syntax then the error stack pointer
; will be an editing routine and the position of the error will be shown
; when the lower screen is reprinted.
;; ERROR-2
L0056:
POP HL ; pop the return address which points to the
; DEFB, error code, after the RST 08.
LD L,(HL) ; load L with the error code. HL is not needed
; anymore.
;; ERROR-3
L0058:
LD (IY+$00),L ; place error code in system variable ERR_NR
LD SP,($4002) ; set the stack pointer from ERR_SP
CALL L0207 ; routine SLOW/FAST selects slow mode.
JP L14BC ; exit to address on stack via routine SET-MIN.
; ---
DEFB $FF ; unused.
Andy