The source code for 64 column printing was originally provided by Andrew Owen in a thread on WoSF. Reproduced with permission.
BASIC normally provides only 32 columns for printing text. This machine code routine, which can be called from BASIC, effectively doubles the number of columns horizontally available for printing.
In BASIC, do a CLEAR 49999 so that BASIC won’t overrun the machine code in memory.
Assemble or load the following routine at address 50000. Do a RANDOMIZE USR 50000, to run the routine and perform the magic for 64 column printing.
To print in 64 columns, use stream 4, for eg: PRINT #4;AT 0,53;”Hello World”

; ---------------
; 4x8 font driver, (c) 2007 Andrew Owen
; ---------------
org 50000 ;
; --------------------------------
; CREATE CHANNEL AND ATTACH STREAM
; --------------------------------
;
; Based on code by Ian Beardsmore from Your Spectrum issue 7, September 1984.
c_chan: ld hl,($5c53) ; a channel must be created below basic
; so look at the system variable PROG
dec hl ; move hl down one address
ld bc,$0005 ; the new channel takes 5 bytes
call $1655 ; call the MAKE_ROOM routine
inc hl ; move HL up one address
ld bc,chan_4 ; could write the bytes directly but
; then code would be non-relocatable
ld (hl),c ; low byte of the output routine
inc hl ; move HL up one address
push hl ; save this address for later
ld (hl),b ; high byte of the output routine
inc hl ; move HL up one address
ld bc,$15c4 ; address of input routine
ld (hl),c ; low byte of the input routine
inc hl ; move HL up one address
ld (hl),b ; high byte of the input routine
inc hl ; move HL up one address
ld (hl),'P' ; channel type; 'K', 'S', 'R' or 'P'
; attach stream
pop hl ; the first address plus one of the
; extra space stored earlier
ld de,($5c4f) ; store the contents of CHANS in DE
and a ; clear the carry flag before
; calculation
sbc hl,de ; the difference between the start of
; the channels area and the start of the
; extra space becomes the offset, stored
; in HL
ex de,hl ; store the offset in DE
ld hl,$5c10 ; store the contents of STRMS in HL
ld a,$04 ; stream number 4
add a,$03 ; take account of streams -3 to -1
add a,a ; each of the seven default streams has
; two bytes of offset data
; the total number of bytes occupied,
; held in a, forms the offset for the
; new stream
ld b,$00 ; set b to hold $00
ld c,a ; set the low byte of the offset
add hl,bc ; the offset is added to the base
; address to give the correct location
; in the streams table to store the
; offset
ld (hl),e ; the low byte of the offset
inc hl ; move HL up one address
ld (hl),d ; the high byte of the offset
ret ; all done
; -----------------
; CHANNEL #4 OUTPUT
; -----------------
;
; Based on code by Tony Samuels from Your Spectrum issue 13, April 1985.
; A channel wrapper for the 64-column display driver.
chan_4: ld b,a ; save character
ld a,(atflg) ; value of AT flag
and a ; test against zero
jr nz,getrow ; jump if not
ld a,b ; restore character
atchk: cp $16 ; test for AT
jr nz,crchk ; if not test for CR
ld a,$ff ; set the AT flag
ld (atflg),a ; next character will be row
ret ; return
getrow: cp $fe ; test AT flag
jr z,getcol ; jump if setting col
ld a,b ; restore character
cp $18 ; greater than 23?
jr nc,err_b ; error if so
ld (row),a ; store it in row
ld hl,atflg ; AT flag
dec (hl) ; indicates next character is col
ret ; return
getcol: ld a,b ; restore character
cp $40 ; greater than 63?
jr nc,err_b ; error if so
ld (col),a ; store it in col
xor a ; set a to zero
ld (atflg),a ; store in AT flag
ret ; return
err_b: xor a ; set a to zero
ld (atflg),a ; clear AT flag
rst 08h ;
defb $0a ;
crchk: cp $0d ; check for return
jr z,do_cr ; to carriage return if so
call pr_64 ; print it
ld hl,col ; increment
inc (hl) ; the column
ld a,(hl) ;
cp $40 ; column 64?
ret nz ;
do_cr: xor a ; set A to zero
ld (col),a ; reset column
ld a,(row) ; get the row
inc a ; increment it
cp $18 ; row 24?
jr z,wrap ;
zend: ld (row),a ; write it back
ret
wrap: xor a ;
jr zend ;
; ------------------------
; 64 COLUMN DISPLAY DRIVER
; ------------------------
pr_64: rra ; divide by two with remainder in carry flag
ld h,$00 ; clear H
ld l,a ; CHAR to low byte of HL
ex af,af' ; save the carry flag
add hl,hl ; multiply
add hl,hl ; by
add hl,hl ; eight
ld de,font-$80 ; offset to FONT
add hl,de ; HL holds address of first byte of
; character map in FONT
push hl ; save font address
; convert the row to the base screen address
ld a,(row) ; get the row
ld b,a ; save it
and $18 ; mask off bit 3-4
ld d,a ; store high byte of offset in D
ld a,b ; retrieve it
and $07 ; mask off bit 0-2
rlca ; shift
rlca ; five
rlca ; bits
rlca ; to the
rlca ; left
ld e,a ; store low byte of offset in E
; add the column
ld a,(col) ; get the column
rra ; divide by two with remainder in carry flag
push af ; store the carry flag
ld h,$40 ; base location
ld l,a ; plus column offset
add hl,de ; add the offset
ex de,hl ; put the result back in DE
; HL now points to the location of the first byte of char data in FONT_1
; DE points to the first screen byte in SCREEN_1
; C holds the offset to the routine
pop af ; restore column carry flag
pop hl ; restore the font address
jr nc,odd_col ; jump if odd column
even_col:
ex af,af' ; restore char position carry flag
jr c,l_on_l ; left char on left col
jr r_on_l ; right char on left col
odd_col:
ex af,af' ; restore char position carry flag
jr nc,r_on_r ; right char on right col
jr l_on_r ; left char on right col
; -------------------------------
; WRITE A CHARACTER TO THE SCREEN
; -------------------------------
;
; There are four separate routines
; HL points to the first byte of a character in FONT
; DE points to the first byte of the screen address
; left nibble on left hand side
l_on_l: ld c,$08 ; 8 bytes to write
ll_lp: ld a,(de) ; read byte at destination
and $f0 ; mask area used by new character
ld b,a ; store in b
ld a,(hl) ; get byte of font
and $0f ; mask off unused half
or b ; combine with background
ld (de),a ; write it back
inc d ; point to next screen location
inc hl ; point to next font data
dec c ; adjust counter
jr nz,ll_lp ; loop 8 times
ret ; done
; right nibble on right hand side
r_on_r: ld c,$08 ; 8 bytes to write
rr_lp: ld a,(de) ; read byte at destination
and $0f ; mask area used by new character
ld b,a ; store in b
ld a,(hl) ; get byte of font
and $f0 ; mask off unused half
or b ; combine with background
ld (de),a ; write it back
inc d ; point to next screen location
inc hl ; point to next font data
dec c ; adjust counter
jr nz,rr_lp ; loop 8 times
ret ; done
; left nibble on right hand side
l_on_r: ld c,$08 ; 8 bytes to write
lr_lp: ld a,(de) ; read byte at destination
and $0f ; mask area used by new character
ld b,a ; store in b
ld a,(hl) ; get byte of font
rrca ; shift right
rrca ; four bits
rrca ; leaving 7-4
rrca ; empty
and $f0 ;
or b ; combine with background
ld (de),a ; write it back
inc d ; point to next screen location
inc hl ; point to next font data
dec c ; adjust counter
jr nz,lr_lp ; loop 8 times
ret ; done
; right nibble on left hand side
r_on_l: ld c,$08 ; 8 bytes to write
rl_lp: ld a,(de) ; read byte at destination
and $f0 ; mask area used by new character
ld b,a ; store in b
ld a,(hl) ; get byte of font
rlca ; shift left
rlca ; four bits
rlca ; leaving 3-0
rlca ; empty
and $0f ;
or b ; combine with background
ld (de),a ; write it back
inc d ; point to next screen location
inc hl ; point to next font data
dec c ; adjust counter
jr nz,rl_lp ; loop 8 times
ret ; done
; --------------
; TEXT VARIABLES
; --------------
;
; Used by the 64 column driver
atflg: defb $00 ; AT flag
row: defb $00 ; row
col: defb $00 ; col
; -------------------
; half width 4x8 font
; -------------------
;
; 384 bytes
font:
defb $00,$02,$02,$02,$02,$00,$02,$00,$00,$52,$57,$02,$02,$07,$02,$00;
defb $00,$25,$71,$62,$32,$74,$25,$00,$00,$22,$42,$30,$50,$50,$30,$00;
defb $00,$14,$22,$41,$41,$41,$22,$14,$00,$20,$70,$22,$57,$02,$00,$00;
defb $00,$00,$00,$00,$07,$00,$20,$20,$00,$01,$01,$02,$02,$04,$14,$00;
defb $00,$22,$56,$52,$52,$52,$27,$00,$00,$27,$51,$12,$21,$45,$72,$00;
defb $00,$57,$54,$56,$71,$15,$12,$00,$00,$17,$21,$61,$52,$52,$22,$00;
defb $00,$22,$55,$25,$53,$52,$24,$00,$00,$00,$00,$22,$00,$00,$22,$02;
defb $00,$00,$10,$27,$40,$27,$10,$00,$00,$02,$45,$21,$12,$20,$42,$00;
defb $00,$23,$55,$75,$77,$45,$35,$00,$00,$63,$54,$64,$54,$54,$63,$00;
defb $00,$67,$54,$56,$54,$54,$67,$00,$00,$73,$44,$64,$45,$45,$43,$00;
defb $00,$57,$52,$72,$52,$52,$57,$00,$00,$35,$15,$16,$55,$55,$25,$00;
defb $00,$45,$47,$45,$45,$45,$75,$00,$00,$62,$55,$55,$55,$55,$52,$00;
defb $00,$62,$55,$55,$65,$45,$43,$00,$00,$63,$54,$52,$61,$55,$52,$00;
defb $00,$75,$25,$25,$25,$25,$22,$00,$00,$55,$55,$55,$55,$27,$25,$00;
defb $00,$55,$55,$25,$22,$52,$52,$00,$00,$73,$12,$22,$22,$42,$72,$03;
defb $00,$46,$42,$22,$22,$12,$12,$06,$00,$20,$50,$00,$00,$00,$00,$0F;
defb $00,$20,$10,$03,$05,$05,$03,$00,$00,$40,$40,$63,$54,$54,$63,$00;
defb $00,$10,$10,$32,$55,$56,$33,$00,$00,$10,$20,$73,$25,$25,$43,$06;
defb $00,$42,$40,$66,$52,$52,$57,$00,$00,$14,$04,$35,$16,$15,$55,$20;
defb $00,$60,$20,$25,$27,$25,$75,$00,$00,$00,$00,$62,$55,$55,$52,$00;
defb $00,$00,$00,$63,$55,$55,$63,$41,$00,$00,$00,$53,$66,$43,$46,$00;
defb $00,$00,$20,$75,$25,$25,$12,$00,$00,$00,$00,$55,$55,$27,$25,$00;
defb $00,$00,$00,$55,$25,$25,$53,$06,$00,$01,$02,$72,$34,$62,$72,$01;
defb $00,$24,$22,$22,$21,$22,$22,$04,$00,$56,$A9,$06,$04,$06,$09,$06;